Do BA have documented their 'official policy' on this somewhere?
Odd coincidence - I actually came here to ask this very question this evening, as 90% of my work travel is with 2 blue colleagues at the moment (meaning I'm having to slum it in Wetherspoon as I can't guest 2 of them in the flounge).

I've always assumed that although you can guest in the lounge, none of the priority airport movement allows guests - and I've marched on through fast track meeting my colleagues at the other side, which has them laughing hysterically at me when they get through normal security quicker than I get through FT.... But recently I was flying with my parents and with me a GCH and my mum an SCH, we brought my (blue) dad through as a group just fine - so I was starting to wonder the same about my colleagues and whether I can waft them through FT security and priority boarding with me.