Hi Jared, I believe the (legacy) "Project Users" relationship governs the Projects I'm On behavior, and am aware of a couple of ways to build up such a guest list:
be the owner of the project (which -- as you guessed -- might have been the case when you created the project in question)
be added to one or more tasks (there is a relatively common convention of creating an "everybody" task for exactly this purpose
interestingly, but As Designed, simply booking time against a project or task is not enough to get on the list
nor is subscribing
nor is sharing
You can see the results under the (legacy) Project Staffing tab, and in some cases (e.g. not the owner, have no assignments), remove someone. Regards, Doug Den Hoed - AtAppStore Got Skills? Lend a hand!
https://community.workfront.com/participate/unanswered-threads