As soon as a geohashing location is given its own page in order to be described, considered for an expedition, or reported on as an expedition, it is classified as a meetup. The term is a bit of a misnomer, as in many cases no one will meet or even try to meet anyone, but as the original purpose of geohashing is to provide opportunities for people to meet, every geohash, even if it's inside the mouth of an active volcano surrounded by pirates and velociraptors, is a potential meetup.
Once someone considers going to a geohash, it remains a meetup, but also joins Category:Expedition planning. And if someone tries to reach it, it's becomes Category:Expeditions. There is currently no particular term specifically describing an expedition where geohashers actually met one another.
The term meetup is also applied to what happens at an agreed alternate meetup location, when the official coordinates are inaccessible.
To see which hashers have actually met, have a look at the Meetup graph - and add whom you have met.