Is there any reason to hang out with people?

As far as I know, this has been (together with the romance part) removed from the game. Hanging out with your friends is just a way to do the 'pastime activities' which also count towards your 100% checklist. But they can also be done alone. And it's a way to get some more back-story through the dialogue.

(Of course you standard perks train while hanging out, e.g. stamina when running, etc.)

As @Chaos_99 has stated, hanging out with friends can help count towards your 100% checklist.

It's also required for the Three Man Army achievement where you must hang out with both of the other 2 characters and attain a 3 star wanted level for 3 minutes.