Group size limit is around 24 or so. It's just that not more then 4 can be in the same instance of any non-trial dungeon at the same time (excess players land in a seperate instance).
The only persons you can get precise position measurement are group members.
Free port does seem to match your guild members position to wayshrines, but I am not sure how precise it's resolution is. Still propably the best part to start.