I've been doing a fair amount of live sound work lately.  Weird twist of fate has had me have 3 a capella groups in the past week.  The third group was, I thought, the best of the 3.  Anyway, after their performance, we chatted and I offered to bring them into my studio to record a few of their songs.  And, as I'm a terrible businessman, I told them it would only cost a couple cups of coffee and a snickers bar.  The group is comprised of college freshman, I might add.  Female group, with a beat box guy who also sings.
 
So, of course, I've never done a session with a group like this.  I am lucky enough to have a facility where I can isolate 4 different performers simultaneously, however having matching mics/pre's isn't an option.   Gear wise, I'm pretty simple with a AKG 414 and a Grace 101.  MOTU 896 is my interface, with an i7 920/24gb ram and running the latest Sonar X3. 
 
My initial plan was to have the group record their song all at once, with the 414 in the middle-ish of them.  Use that as a scratch track, then bring back individual/duo/trios and add that in while slowly taking out the scratch track.   I'm really in love with the Grace/AKG combo for recording vocals and,  I know me,  I probably won't be satisfied with using any other mics, like Beta 87, Beta 58, etc. 
 
Anyway, that's where I am at in my initial planning stages and crazy quirks that I possess.  Thanks in advance.

Put the 414 in Omni and let 'em rip.

I'm in agreement with Mike.  Acapella groups tend to have a pretty good sense of levels among each other and the live take of the group on an omni mic will pobably sound awesome.  If there is a voice or two that seems to over or underwhelm maybe mic a couple of them  bur i suspect that person(s) would be out of the group if that were the case

I am liking this option as it's obviously very easy.  I'm not sure how I can deal with such little control, so I'll trust the singers.  I can't even hum in key.  Thanks guys!

I've recorded and live engineered unaccompanied singers quite a few times. And done some such singing years ago.

I'd go with an omni mic as suggested. Then in case stereo spread is needed I'd add a couple of more distant mics set up as a stereo pair, or set up overhead, a bit like drum overheads. That's how choirs are often miced live.

The fun starts if you find you need to EQ, compress or de-ess some voices and not others. In which case I'd suggest close-micing each singer with a directional mic. Be wary of isolating them from each other. There's usually a great deal of visual interaction between unaccompanied singers, even if it's not noticeable to an outsider. You can say a lot to a person you've worked with closely by how you raise an eyebrow or how you look at them.

Singers are also often used to responding to each other's performance, volume etc. Some work by literally singing at each other and get necessary feedback, interaction and energy from that. If Bill is always used to Fred to his right, Jane to his left and their distinctive volumes compared to his own, setting up an individual monitor mix for Bill can be a nightmare. And Fred and Jane will probably also need their own individual monitor mixes as well.

Setting all this up can take quite some time and result in bored or frustrated singers if they're not used to recording and spending a large part of a session just getting to the point recording proper can actually start.

TLW, I believe that your suggestions are going to be the best way to roll.  I had the group into my studio on Tuesday, great kids for certain.  So full of energy and excited to have this opportunity.
 
The studio rat in me wants to isolate alto 1 and 2 and soprano 1 and 2, however thank you so much for the insight as you are correct to the group's interaction being important to the performance.  I'm thinking of using the 414 on the soloist, and a mix of perhaps some condensers (akg c430) or beta 87/beta 57/beta 58/sennheiser 935.  Obviously I do a lot of live audio work.  
 
Perhaps I can get a solid enough recording in one session, and then bring back a few of the more talented ones to add a few more layers, or possibly touch up the lead vocals.  I must remind myself to try and do everything to squeeze the best original performance out of this group.
 
 
 
 
 

I've often found the best way is with an Omni mic in the middle, just try to get them sing naturally and gently remind them that they can perhaps not have the freedom of movement  that they're used to.
Personally, I also like to have them sing their parts over the top separately, so you can use these to add to the performance. This may not be an option if they are not used to working this way.
You may even find that the individual parts comprise a different, more controlled version, that may or may not be what they prefer. Then you have the option of adding the Omni version underneath to add some more flavour.
I just like to have as many options as possible, though this is not always a good way to work as it can give uncertainty to the end product.
I feel like I have ducked and weaved around your question, but it's horses for courses and you may find things sort themselves out when you start recording and you get their reaction to the songs as the session progresses.

You are definitely going to miss out if you don't record in stereo.  Can you rent another 414 or a matching pair of other mics?
 

Oh man, I never thought about doing a stereo option.  That will teach not to check this forum everyday!  Sorry!
 
So here's the update.  I think we're done with the 'main' tracking of vocals.  I had the group in my little studio form a semi circle similar to their performance stance---albeit a little smaller semi circle than normal.  I put the 414 on omni and had it near the lead singers.  Off to their sides were two ladies each, and I had them sing into a AKG C430.  Used an ART Preamp on them.  Then, fanning out, next were Beta 57's to capture either the 1 or 2 ladies.  It seemed like the best pair I had on hand for the task.  I gave the beatbox guy a beta 58.  
 
We ran through one song to check levels and adjust mic placement and we were off.  2 takes on 4 songs and done.  2 of them are coming in tomorrow night and we'll review and start the mixdown and overdub process.  One of the girls that's coming in sang lead on 3 of the songs, and I was, at times, awestruck.  A soprano with a huge range and can effortlessly get up to the high D.  
 
I know I went against the 'suggested method' however with last week's dry run using the 414 in the middle of the room I was not satisfied with the levels that all the girls gave me.  One of the lead vocalists is fairly quiet, one very loud, and the other right in the middle.  
 
I can't wait to get a rough mixdown or something posted to get opinions on all the wonderful helpful folks in this forum.  
 
Let me tell you---the look on these kids faces was priceless.  They felt like they were so important and are so proud of what they do, and just love to sing.  I am more than honored to have the ability, time and resources to provide these folks with a quality representation of their passion.

The sound of vocals blending in the air is one of the sweetest sounds there is.
 
I'm glad to think that you had an enjoyable time working with them. Wishing you many happy returns.
 
best regards,
mike