I own a number of groups, the larger one is just about 5000 users. All hobby-related, some have been in existence for nearly 20 years. I can't see the group membership not falling drastically if members are required to pay individual annual fees. [Mod Note: Again, current groups are considered legacy and their pricing will not change.]
For my groups, I'd rather occasionally beg for donations than have each member deal with this individually. A simple tiered structure could work based on the number of members, something like this:
0-500 members = free (or free with ads)
1000 = $
2500 = $$
5000+ = $$$
It would be good to get a discount for pre-paying for a number of years. I'd rather fund-raise once every few years than every year. As the group exceeds a tier threshold, an additional payment may become required. This seems simple enough for anyone to understand, and the pricing and tiers can be adjusted over time.