I'm not sure what criteria I could use because promotion to my Premium Membership is already based on their initial forum participation (such as posts, likes, etc) and it's some of these members who initially earn their PM then turn into lurkers.
Perhaps I just don't fully understand how to use usergroup promotions, but as far as I can see the criteria I can use is limited to what's on the "Apply this promotion while..." tab in Create New Promotion. Under this tab, I can select various Content & Achievements such as: must have posted x amount, received x likes, etc and because I have Showcase and Featured Threads addons there are several other criteria's I can select like: user has at least x threads featured. However, I don't know how I could apply a new criteria to my premium members who have already achieved all these?
If I'm misreading how usergroup promotions are supposed to work please feel free to point me in the right direction - still finding my way around xf, cheers.