Several of the FlyerTalk airline forums have threads, often stickied, where FTers may meet others at or near the airline club location so that a club member may host a non-member. (See, as examples, the Delta (link above), Continental and United forums). The participants in those threads obviously don't think it's tacky.

As to whether non-F guests are allowed in a lounge for F-class passengers, will be up to the airline involved.