You can join either as an individual or as part of a team. If you're looking for a team to join, check out the Discord server provided on the page. There, you can let others know you're searching for a team and find one to join.

From my understanding (I usually just work alone, but) when working on a team you usually have like one representative for the team who uploads the game to one place and then credit everyone on the upload site or by linking to them on the submission page/form/etc. Otherwise it would look like everyone on the team submitted the same game multiple times which would be confusing for judges and people trying to play your game because the same game would come up multiple times in search/submission feeds.