Limit records shown using "does not contain logged in user"

I have a field in a record which allows for multiple entries of users - "READ BY".

I want to be able to show a table containing all records where "READ BY" does NOT contain the currently logged in user.

I am sure I've done this once, but I deleted the page it was on, and I cannot get this functionality back.

I am trying to implement the ability to mark a record as having been viewed (or read) by the logged in user, and I want to be able to show the records that they have NOT read in order to mark them as read.