Q: What is the limit of Windows Live Groups I can have or Join? How many people can join my Windows Live Groups?
A: Windows Live Groups allows you to create or become a member of up to 40 groups. For example, you can create 20 groups that you are the owner of and be a member of 20 groups.

Each group can have up to 1000 members. However, if a group has Group conversations integration in Windows Live Messenger enabled, it can have a maximum of 40 members.