In L2TeleporterInstance, or your own instance:
1. add special container for those players, for example:
2. create boolean to check conditions of the party (like members level, if they are close to the GK, if party != null, members count and so on, sample:)
3. create new bypass for this kind of teleport, for example tele_party
Inside of this bypass, use already created boolean:
4. Create new inner class for new task, which will tele players back after certain time.
5. Again inside bypass under teleporting players, use this, to run task:
And put there time in milisec, after this time they will be teleportedback
I could make some mistakes, i was to lazy to check it with eclipse, but will be ok.