In American English: "On a team" means being a member of a team. It refers having been officially placed *on* the roster listing the team's players: - Sam's so happy about being on the basketball team. "In a team" means being in place with others for the purpose of working together: - The six horses were harnessed together in a team. "As a team" means working with others to accomplish something. - The volunteers worked as a team to put the fire out. "For a team" means for the benefit of a team: - That new forward is doing great things for the basketball team. - Sam's father baked cupcakes for the whole team. "At a team"... I can't think of it making sense as anything other than "in the direction of": - The cheering crowd threw handfuls of confetti at the team.

i think it because he will be a part of the team ,so wee use in because team contain him this what i understand anyone for correct if there is a mistake ?

(In a team or for a team) are used in British English while (on a team) is common in American English. At a team?! I've never heard or seen.