Penny For Your Thoughts

The prerequisite quests in order to get Penny Clobberbottom as your follower are:

Tangleheart
-- Growing Wood

Lost Lumberjack
-- Chapter I: Plant Food
-- Chapter II: The Harvest
-- Chapter III: Ritual of the Charred

Once you complete these quests, you'll see a "!" on your map at Beastwatch.

She is located in Beast watch, but can only become your follower if you choose to make The Lumber Mill from the quest Your Base, Your Choice

I accidentally made the sparring arena without researching first.. Penny sassed me and said the arena "isn't really her style" and that I could come back later for a costly re-build. Lesson learned: read up first..
As a side note, I have not completed the prerequisite quests yet. I was in the area early and got the "your base, your choice" before I realized I would be making a choice between outposts and followers.