I mentioned Amsterdam, both in jest, and seriously.
Do we want to use an airport hotel at the weekend, on the basis we should be able to get a cheap block booking? (and soundproofing) (as opposed to an "in town" hotel)
Pros:
International Hub airport, served by international high speed rail links, with local 15min train service to Amsterdam itself - "Central" location for Europe.
The many attractions of Amsterdam itself. (Think of our extra curricular interests - I'd hazard a guess that all can be met)
"They all speak English" - Helpful for monolingual Americans, and being Europeans, unlike Brits, probably are fluent in several other languages for our European Chums.
Cons:
Costs as yet unknown vs Other European candidate locations.
Critical mass of local, on the ground staff unknown as yet vs Other European candidate locations.
So, what are the requirements? Who are we catering for? How do we end up with the lowest weighted cost for attendee's given their number, and where they are travelling from?
I think Brussels is another cheap major hub - obviously direct flights from US makes the whole process easier for the US contingent.
If it can be held in an interesting tourist location, then it may be possible to pitch the trip as a starting point for a European Vacation for those that need to justify a European trip ...
Anyway, just some some thoughts