Dear OP, I hate to be the killjoy, but if you want a cast-iron guarantee of being able to access the lounge with all members of your party then the best way is to ensure that everyone is eligible. So upgrade one of them, or something like that.

When I worked in Heathrow Customer Services I often worked front of house in the lounges, or by the door in CCR. On a busy day, in the space of a couple of hours I'd have at least a dozen people trying to sneak in at least one other traveller who didn't have the status or ticket that entitled them to get in. Often times it was a case of trying to get in more than one person.

Some people tried the charm way, some tried the sneaky way, some tried to bully me, some tried to bribe me. If I let every single one of them in, those lounges would've gone from busy (especially North Galleries) to 'sardine can' level, and that would've been unfair towards all those who paid a pretty penny for it.

I know full well that you're only asking it once for one mate, so did the other 10 people before you...