One could expect a son/brother etc to have the same surnames. Females obviously are more likely to have a different surname. Technically thus Lounge staff could refuse them access - but as I said, unlikely and I'd be pretty surprised if that happened.
I worked in the London Lounge for 2 1/2 years and you just get a gut feeling when you see 3 guys in business suits on a Friday night who look nothing alike and don't seem that close trying to claim they're related. Families just have a different way of interacting - even if they're not getting along!