I have been into an officers mess for an official Sqn guest dining in night as being a member of the squadron association. An invite was sent out and we put in requests to be considered. I was on a course up near where the squadron was based. 4 other members of the association attended whom were all like myself, non military ID holders but were Officers on their time on the unit, to my non commissioned time. We of course signed onto the base and escorted to the mess where we had booked rooms and did not leave the complex the entire time until escorted off. This was only 5 years ago. Of course it was booked in advance and we were closed as a visit like any other. Depends on the commander of the Station and other things now, but does depend on sponsors and escorts. One officer was there that did question why I was there (due to my previous military ramk) to which my reply was "Because I am Mister." I found out later the colourful history of the Officer in question,.