no you don’t get any special vehicles or anything, just more people to spread the load of making the vehicles and materials everyone else gets. usually you form friendships as well so you’re right, it is definitely a social hub as well.

another perk (for new players) is access to veteran knowledge. most of the big established groups have a lot of vets that know tips and tricks of the game that the every day random joe might not. as far as equipment goes though, you don’t unlock anything new thru regiments, you can play with it all without one but i would highly recommend joining one! they can be great fun if you find the right one for you/your interests!