Hi @Nganthoiba.Kha,
Currently there's no way to do this, and users would need to sign up to each team to be able to talk to each other.
We're looking into switching this behaviour, and making it so users have only one account per server that they can use to sign in to different teams. This would allow Direct Messages to be system wide so it's possible to send a Direct Message to anyone on the server, even if they were on a different team.