A FlyerTalk Posting Legend
Join Date: Sep 2009
Location: Minneapolis: DL DM charter 2.3MM
Programs: A3*Gold, SPG Plat, HyattDiamond, MarriottPP, LHW exAccess, ICI, Raffles Amb, NW PE MM, TWA Gold MM
Posts: 100,417
IIRC there was a DO a few years ago that organized a control tower tour at a major USA airport. They discovered at the last minute that USA citizenship was required.
So the OP should be sure to check this issue early if any of the work people to be invited might not have USA citizenship and photo ID documents such as passports to prove it.