How to let users add value to a list field of type "link to another record"?

in the airtable i have a table contains a field of type “link to another record” that allow liking to multiple records, this field is liking to users table, and I want to let users add value (the user add himself) to this field without clear this field value ( without deleating athor users stored in this field

Also looking to do this. Essentially I have a “Playtest” portal, and I want people to be able to sign up to the playtest (add their User into the Signup column in the Playtest row).

How can we do this?