Apologies, I'm sure this is discussed but search function seems to not be working well this afternoon.

Si I'm wondering how I would turn off multiple instances of a plugin.
For example I'm using Waves NLS on every single track, I'd like to be able to turn them all off with one click for instant side by side / rolling comparison.
I'm sure it is possible using something like the grouping function, but can't seem to get it to work.

Thanks,
Adam.

The only way  know of to switch multiple (but not all) FX in and out is by using Mix Recall. Bypass button grouping would be handy (and faster) if it were possible. There might already be a feature request for it; if not, you should enter one.

you can CTRL select (or drag and select) to select the tracks you want (to bypass FX).
then while holding down CTRL click the bypass button on the FX bin.
this will by pass FX for all selected tracks.
this wont of course let you cherry pick which FX you want to bypass, but it would allow you to bypass that tracks FX.
 
other than that Dave's idea above is probably a better route.
not as fast, but more detailed and accurate.
 
there's a few ideas from both of us to get you started