Then another option might be to make a forum that only that group can see and make a post telling them what to do. Then it's up to them to apply to the group or not. That will save sending out 1000's on PMs and associated notification emails.
-:|:- Support Request Template -:|:- "Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb"
Yes that might be a better strategy and might as well use a closed group. The group leader is going to be busy at first approving users anyway.
I considered just making all group A members also Group B members and requiring them to opt out if not wanting to be part of it, but the 'politics' of it make a conscious decision to be part of the discussion in the associated forum better I think.
Thanks for crystallising my thoughts.
'And what is good, Phaedrus, And what is not good—Need we ask anyone to tell us these things?'