From what I remember I think that the levels proposed by @PeterWolf work well. I would just add in the fee structure.
I'll let you decide if we collect the fee immediately or after 30 days. Clearly if we're purchasing a license for Yammer/Socialcast then the fee has to be paid immediately.
I would further propose we have a renewal fee -- for people who are continuously members -- as well as a higher fee for reinstatements for anyone who decides to drop and then wants to re-enter.
Be happy to hash this out with you, Peter, etc some night if you like.