I've been an occasional guest in the Diamond Lounge here at the local Horseshoe . Each Diamond can bring one (maybe two?) guest(s) along, who will get all of the free drinks and free food and such, when it's available.
You are allowed one guest. You can decide who on each visit (LOL!) She can not enter without you though.
If you're talking about Vegas, 1 guest. I think some of the ones in AC have entry fees for anyone below 7* but never been there myself..just from reading what other people say.
In AC, a Diamond player with 25k tier credits can use a Diamond Lounge with 1 guest. If you have less than 25k tier credits, the charge is 1000 reward credits per person. The 25k tier credits can be in the previous year to earn your Diamond status or year to date.
if the room is not packed sometimes they will let you bring more than 1 person, you can also go to the vip desk and get a 1 day pass to bring someone in if they deny you at the lounge door
Now one thing I just remembered, which may have changed but I'm not sure. The last time I was a guest at the local Horseshoe DL with a friend, the lady at the desk asked for MY TR card when we signed in. I don't remember that happening before.