Checkboxes available for all members, updated individually

I am working on a website where inside the members area there will be a list of 20 or so items, all available to tick when complete. I have set up the members area and database fine, I have a table called "users" for the members, and a table called "offers" for the offers they can tick when complete, this information will then need to be saved. My question is how do I implement the checkboxes within the database? Am I going to need another table which links the current session user to each offer? Some kind of array? I can't quite get my head around it!