My setting has a town, kind of like the one at the entrance to the Grand Line in One Piece, where most people going on an adventure have to stop. The Townspeople there know adventurers do better in parties, so they guide any visitors to the local tavern to help them form groups, and so that they can get gossip about them, as nothing much else happens in this small mining town. The tavern is run by a former adventurer that tells them about the first choice they’re going to have to make in their journey, which path to take through the mountains.
So it’s not me that was pushing them together to make a party out of nowhere, it was nosy locals in setting. This also let me introduce a recurring rival character who refused to join their party because he’s a rich asshole.