How do you join the Brotherhood of Steel on Fallout

Once you find their HQ, Cabbot will give you a mission to go to the Glow (he will mark it on your map) and retrieve an artifact. You will need anti-radiaton drugs and a rope. The artifact you need is on the first floor, it is a holodisk and its own description will tell you it is the thing you are looking for. However it's worth exploring the old base thoroughly for some nice items and interesting background info.