GroupMembership.cs

public class GroupMembership
{
    private const int MaxMembers = 100_000;
    private readonly HashSet<Guid> _members = new();

    public Guid GroupChatId { get; private set; }

    public GroupMembership(Guid groupChatId)
    {
        GroupChatId = groupChatId;
    }

    public void AddMember(Guid userId)
    {
        if (_members.Count >= MaxMembers)
            throw new InvalidOperationException("Too many members");

        _members.Add(userId);
    }

    public void RemoveMember(Guid userId)
    {
        _members.Remove(userId);
    }

    public IReadOnlyCollection<Guid> Members => _members;
}

public class GroupChat
{
    public Guid Id { get; private set; }

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

    public void PostMessage(Guid senderUserId, string message)
    {
        // Assume sender membership is already validated externally
        // Here we handle domain logic related to messaging
    }
}