Thanks for that clue! I had misunderstood the purpose of the first field in the string. I had been using the “members only category” associated with group membership.
Once I changed the first field to an existing, public, open-to-everyone category, everything worked as expected. 
news~Join Group X!~Come join our formerly top secret group.~/pub/membershipinfo~group-x
TLDR (for how my brain interpreted it) - I needed to understand that the first field is where you want the “preview” advertisement to appear in the category list. (In this case, just above the public “news” category.)