I have never heard of any limits on the number of users create or the number of users added to any given group.  In fact, since the Everyone group has every user as a member, that points to no limit (or SQL and/or OS/hardware limits and will be exceedingly hard to reach) on the number of members.

The only limits I am aware of for users are:

• A user account name can consist of any combination of letters, numbers, and symbols except the backslash (\), at symbol (@), or colon (:). Additionally, the dollar sign ($) cannot be the last character in a user account name.
• A user account name can have a maximum length of 47 characters.
• A user account description can have a maximum length of 79 characters.
• A password can have a maximum length of 31 characters.

The only limits I am aware of for groups are:

• A group name can consist of any combination of letters, numbers, and symbols except the forward slash (/), backslash (\), at symbol (@), or colon (:). Additionally, the dollar sign ($) cannot be the last character in a group name, and the period (.) cannot be the only character in the name.
• A group name can have a maximum length of 47 characters.
• A group description can have a maximum length of 79 characters.