Adam joins the TARDIS crew by Rose's invitation at the end of the "Dalek" story. In the next story, "The Long Game," it turns out that Adam is strictly out for number 1 and doesn't help with the problems the Doctor is trying to solve, so at the end of that story the Doctor un-invites him as a TARDIS crew member. This was done on purpose by the writers to see what would happen if one of the companions turned out to be a bad apple.
Adam Mitchel was played by played by Bruno Langley (born 21 March 1983). Bruno seems to have had a successful acting career both before and after his one-night stand on Dr Who.