Welcome the Technical Support section. Help us in assisting you by providing us with a concise and descriptive elaboration of your issues. Be specific and if possible, provide us with a step-by-step instruction in replicating your problem.
How to limit the number of members in a private group?
Jomsocial doesn't have this feature for now :) but this is a good idea if you can limit your group member :) you could suggest it for the future versions in our
uservoice
and if you're in a hurry, you could have oDesk developers custom-code a solution for you
jomsocial-developers