Check-In

I'm trying to create a check-in process to allow users to press a button to check-in. I'm new to Appian and i'm not sure how to go about this. So far what I have is a Meeting record displaying all the details about the meeting, including a user group picker field for attendees. Now what I am trying to do is configure a button that a logged in user can press so that that user can be added to the Attendees field in my meeting record