Today I went to the FDS at Eranin 2 and saw another group player there. I chose the Eranin faction and he appeared red on my radar, so was fighting for the Federation. Neither of us attacked the other (probably just as well as he was in a Cobra and I was flying my rusty Sidewinder).
We didn't communicate with each other we just went about our business shooting NPCs. So fair play to that Cmdr (I can't remember his name).
However I think that there should be either a comms word to use in that situation, I'd suggest 'Peace' as a signal of intent or that there should be a hard rule of the group, either anything goes in FDS or it's not allowed at all.
I'd prefer it not being allowed personally, I have enough trouble shooting NPCs without the added complication of watching out for another player attacking me. If I wanted to be attacked by another player I'd have stayed in the All Group.