oh I see.
Well in that case, to tell you the truth I think fb can be a solution.
For example, if I 'm already in an asso group, I would leave it only in a case of a friend who would not just invite but having a little chat about that.
For example
"Hi Mat, i can see that you 're in an asso group. Don't know if you 're ok with that but our asso is looking for a good active manager and I thought that might you 're interest to join us.
blah blah blah"