no such thing as a stupid question but here´s a possible answer. Let´s use colors as example here.
Your app has a table called Colors with the following entries:
Blue
Red
Black
Green
Yellow
White
Your User database has a field called Color (type Color and checked as list) so the users can add their favorite colors to their profile.
Current User has these colors on their Color field:
Blue
Red
Black
And you want to find out what other colors exist in the table Color that the user hasn´t added to their list. The result you are looking for is
Green
Yellow
White
This is how you set up a search to find those 3 colors that aren´t on Current User´s Color. I will be using a text element for this example:
In your example, instead of colors, you´re talking about Users that have been added to the Friends List (or whatever you are calling it) but the same logic applies. You want to do a search for all Users or Friends that aren´t on Current User´s Friends List. So set your search constraint as I outlined above.
Here´s an invaluable post for understanding more about searching:
http://forum.bubble.io/t/sql-joins-vs-bubble/25806
Hope this helps