Join Date: Aug 2011
Location: Barcelona, London, on a plane
Programs: IB+ Gold, TK E+, Hyatt Globalist, Marriott LTP, IHG Diamond
Posts: 14,181
You have to be registered guests to have access to the lounge.
That said, if your hotel doesn't staff its lounge entrance, but instead just allows access via keycard, then you can quite likely bring in guests without anybody saying anything. (as long as you are comfortable with that rule-breaking)