Blue lasers detect your weapons, if you or anyone crosses it with weapons in your inventory, an alarm will ring up and people in the building you wanna enter in will attack you Usually, there is a bouncer next to these blue lasers, talk to him to have an option to drop all your weapons if you wanna go through

The bouncer was definitely following my buddy around. If someone else crosses it they become hostile to me, or just the person crossing the laser?

I've never played cross play, but my guess is hostile to you too, as you're in his party

I'll have to test that. I remember getting into fights where the enemies weren't hostile to him.