Yes, you can add a guest to the Guest List, even if they didn't buy a ticket.
First, you must first invite your guest directly through Partiful. That guest will appear as Invited on your event Guest List.
Then, you or any cohost or check-in staff can manually check them in by changing their ticket status to Going.
Note: while this guest will appear as “Checked In,” they won’t have access to a scannable ticket or a QR code.