Usage of "on" in the phrase "on our team"

Prepositional usage is idiosyncratic, with even apparently near-identical structures not being similarly available.

Both on our team and in our team are grammatical and used; on our team seems to be the more common choice according to these Google Ngrams (though the two seem equally common in British works).

There are over 300 000 Google hits for both everyone on our team and everyone in our team (though Ngrams paint a different picture). But I'm quite familiar with this type of post-modifier; neither prepositional usage can be considered incorrect.

You are right, a lot of non-natives would prefer "in". But English says "on the team / on the staff".

"On + person group" meaning someone is a member of this group is a special niche of on every learner stumbles on.

Why this curious use? Some uses of on are really hard to explain. I can only give my guess. The underlying idea seems to be "someone was elected or taken on as a member".

Ultimately you have to learn this special use of on.