Head to the house south of the Inn and talk to the Village Chief. Answer yes to gettting rid of the monsters in the tower. The chief says he'll send Linn with you to go through the mountain trail, so now go talk to Linn (she's in the same home, down in the side room).
Linn will say how she misses Mazenda, but she won't join your party... yet...
So, warp back to Northwest Town, and go to the Inn and talk to Mazenda. Once Mazenda joins your party, warp back to the Mountain Village and talk to Linn again, and now, she joins your party!
Now, before you go on, you should have enough gold to go to the Item Shop here and upgrade Linn's gear. (Bront, Bruce, and Linn will be your main attackers for awhile, so it never hurts to get them the best equipment available.)
When you're ready, head to the south of town to reach the entrance to the Mountain Road.