GroupChat.cs

public class GroupChat
{
    private const int MaxMembers = 100_000;

    public Guid Id { get; private set; }
    public int MemberCount { get; private set; }

    public GroupChat(Guid id)
    {
        Id = id;
        MemberCount = 0;
    }

    public void AddMember()
    {
        if (MemberCount >= MaxMembers)
            throw new InvalidOperationException("GroupChat is full");

        MemberCount++;
    }

    public void RemoveMember()
    {
        if (MemberCount == 0)
            throw new InvalidOperationException("No members to remove");

        MemberCount--;
    }
}