The BuddyPress Limit Members Per Groupplugin gives you the flexibility to set the limit for allowed number of members for all groups on your Site or just for individual groups.
Features:-
- Allows you to Limit the Membership globally(for all the groups)
- Allows Site admins to give the group admins capability to override global settings ( If allowed in admin, it can be turned off for individual group)
What it does:-
- Limit BuddyPress Group joining for public groups
- Limit Request Invite/Joining for Private BuddyPress Groups
- Limit the acceptance of group membership requests by member(if a group reaches its specified limits)
Screenshots:-
Admin Settings:- Under BuddyPress->settings you will see the options like this 
You can set a global limit, the error message and also allow/deny group admins to override settings there.
Group Settings:- If the override is allowed, here is what a group admin will see:- 
A group admin can set his/her own limits and also turn off the limit if they want.
Now, we will see some of the examples on how it works:-
Here is a screenshot from directory pages:- 
That is the message shown to the user when they try to join a group which has reached its limit. If the user tries to join from Single Group page, here is what they see:- 
For private group request membership:- 
And if the user got an invitation for a group but before he/she could join, if the group reaches the limit, here is what he/she will see. 