Function for checking up two lists against each other?

Function for checking up two lists against each other?

I have to List's.
List<User>
and
List<Friends>
In User are 10 Coloumns, one of them is "UserID", in Friends we havbe 4
Coloumns, one of them is "UserIDActive".
Now i Wanna have a new List in which only all Users are in it, which are
also in Friends. Connected through the Coloum UserID == UserIDActive with
have the same value.
How to do that? With .contain I can only check two Lists with the same
objects in it.