AddMemberHandler.cs

public class AddMemberHandler
{
    private const int MaxMembers = 100_000;
    private readonly IDbConnection _connection;

    public AddMemberHandler(IDbConnection connection)
    {
        _connection = connection;
    }

    public void Handle(Guid groupChatId, Guid userId)
    {
        using var trx = _connection.BeginTransaction();

        var memberCount = _connection.ExecuteScalar<int>(
            "SELECT COUNT(*) FROM GroupChatMembers WHERE GroupChatId = @Id",
            new { Id = groupChatId }, trx);

        if (memberCount >= MaxMembers)
            throw new InvalidOperationException("GroupChat is full");

        _connection.Execute(
            "INSERT INTO GroupChatMembers (GroupChatId, UserId) VALUES (@GroupChatId, @UserId)",
            new { GroupChatId = groupChatId, UserId = userId }, trx);

        trx.Commit();
    }
}