Wow
For the record, I had trouble, too--
I started making a plant list a while back, set all my categories to 'private' when I set them up, added a few plants to each supposedly private category but only one category was actually private. I couldn't fathom how to get the others to be private and I had no idea what I did to get the one category to be private and not the others--so I bailed on the effort 'til I saw this.
Just went into the manage category page and edited each category--each was listed as private so I didn't have to reselect that, but I did click save changes for each category, and just now when I checked from my partner's page I see that it was successful!
Now it looks like I have no plant list again instead of a barely started plant list
not sure why it really matters
I guess I just wanted to 'keep it private' until I had a more respectable plant list since one of the first plants that I put in was marked 'used to have' along with a note to self about how I killed it (accidentally, of course)!