Easy way (without any sneaking):
1) First go left and free the miners by destroying the totem (and get the password for the door.
2) Split the group and let one char with invisibility scout the area until he finds the second portal. Walk back to the group.
3) Leave the mine with the whole party.
4) Teleport the whole party to the second portal in the mine.
5) Follow the southern way to the east.
This way you never have to sneak once.