Brain boiling from MySQL - How to do a complicated select from multiple tables?

I have two tables: Users and Groups

In my table "Users", there is a column called "ID" for all the user ids.

In my table "Groups" there is a column called "Participants", fields in this column are filled with all the user ids like this "PID_134,PID_489,PID_4784," - And there is a column "ID" that identifies a specific group.

Now what i want to do, i want to create a menu that shows all the users that are not yet in this particular group.

So i need to get all the user ids, that are not yet in the Participants column of a group with a particular ID.

It would be cool if there was a single mysql query for that - But any PHP + MySQL solutions are okay, too.

How does that work? Any guesses?

UPDATE:

i know, that's not code, but is there a way I could do something like this that would return me a list of all the users?

SELECT * 
 FROM users, groups 
 WHERE groups.participants NOT LIKE '%PID_'users.id'%' AND groups.id = 1;
 

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