Transgender women (MTF), just like women, can't play in a men's football team. Transgender Men could, in principle, but as far as I'm aware there are no universally accepted rules on the issue. In the UK, at least, there are two basic requirements: firstly, that trans* players have similar hormonal levels to a typical biological player of the same gender; and, secondly, that the player have some valid ID under their current identity. It's also a matter for the club, and things are complicated by the existence of non-binary players.