Are you on a bowling league or in a bowling league?
Both are correct.It's a matter of context, however both could be used interchangeably.For example:"How many players are on a bowling team?""My friend Aaron and I were on the same bowling team.""At my previous high school I bowled on a team that won the league.""I bowled on a team that won all the league awards."To help stress you may not be the only one, examples could be:"In bowling class, we bowled in teams of three.""There were five people bowling in each team.""Do you prefer bowling in a team of four or five?"