I tested it some more.

The mage portal is only "valid" when the mage is in the same area. If the mage leaves the area the portal becomes unuseable.

test:
- while in heroics with lfg tool
- do portal
- make leaves through portal
-> all others can't use portal
- Mage goes back to instance with Teleport to instance (LFG tool)
--> portal becomes useable for the others again.


---> Resume: Mage needs to be last to go through portal or it becomes useless for others.