Some additional information:-
After playing round with the Mumblelink data ( https://wiki.guildwars2.com/wiki/API:MumbleLink ) we've noticed something.
There are at least two distinct maps for the Gilded Hollow guild hall: Map IDs 1101 and 1121.
These aren't separate instances of the same map, other maps we tried all had the same Map ID regardless of instance, for example Lion's Arch is always map 50.
These are entirely separate maps. For the same place.
This would explain why sometimes we get "This instance will close in 5 minutes" and can swap, and other times don't, but are still in separate versions of our Guild Hall.
Still doesn't explain why we get multiple instances for eight people instead of just one, thought, if a Guild Hall really can't copy with eight people then something is seriously wrong!
So, step one would seem to be to get rid of the duplicate Maps, and just have the one Map ID for Gilded Hollow.
Step two would then be to only have the one Instance of that map per Guild, so everyone ends up in the same one.
Thanks,
Bugrup