"Assigned to" section not showing all users

Hi all! 
I'm very new with Quickbase, and have a quick question. I'm a participating user in an app created by a team member. Within the app, I tried to assign myself a task but I'm not on the "assigned to" list. Does anyone happen to know how to add myself to the "assigned to" list? 

Thank you all! 
Kendyl Brockman