There is a passageway between F and G, so no security worries. If you are an AC *G, then, as I understand it, you can access *G lounges, including UA ones, if you have a boarding pass on any *G flight departing that day from that airport. This would mean any of the UA clubs. And you can invite a guest as long as the guest is also carrying a boarding pass for any *G flight departing that airport on that day.