Can't help you directly with your question (have never taken a guest into a lounge), but have a piece of related advice.
If you are flying from Heathrow you should be given an invitation to the Virgin Clubhouse (that's okay).
Of you are flying from Gatwick (or from another airport and are given a pass for a different lounge), then don't ask the check-in agent for a pass to the Virgin Lounge instead...!
I did this at Gatwick (whilst flying with a Continental BF ticket on a Virgin codeshare) - they gave me a pass to the President's Club instead - and got a long (not too quite), embarrassing lecture from the check-in agent. I did ask very politely and nicely. If I didn't feel so embarrassed and like a criminal (that's how the agent made me feel), I would have been very angry at being spoken to like that. Don't know what the people in the neighbouring lines thought, it was very embarrassing...
Still, happy ending, we went to the Virgin Clubhouse and were welcomed in without a question or a comment.
So moral or the story: don't ask nicely at check-in, just go to the lounge and smile sweetly there. You live and learn.
Boo