Thanks for your help. The party.mUsers.Count(); was a type, it's supposed to be group.Users.Count();. I do take the other parts into consideration, like you said, checking to see if the user is already in a group, but for now I want to just get this Group.Users.Count(); working.
So like you said, I should promote it into another field in the class. Did you mean something like this?
Code:
public void CountMembers()
{
Users = new List<User>();
int count = Users.Count();
}
I will post more code once I get home.