I went to hiden valley after Benny run to the fort and u meet his protectron at his sercet room. They started firing at me.. How do I get to join em?

Keep exploring the bunkers in hidden valley until you enter the one with the BoS base inside, there are three bunkers in hidden valley so keep looking. (If you are at the right point in the main quest an event will trigger and let you in, if not you will need to pick a 100 lock)

First of all make sure you are not wearing faction armor. You can lock pick to get it. Have Veronica get you in, or find one of the three dead BoS scouts in the wasteland. There is one at the top floor of the REPCONN HQ. Grab this holodisk. That disk has a password to get it.

And be prepared to do a long mission that takes you several places. You have the option of removing the leader of the BoS too and replaceing him w/ Hardin. I did that and he finally let me join after I had to kill the Van Gruffs. Which was a pain in the tookis.