Are you on a bowling league or in a bowling league?

Answer 1

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?"