SQL Server 2014 :: Using Data From Two Tables In A Single Script?

Basically I've got two tables with a common column (in this case it's UserID)

The structure of the table Users is basically:

UserID FirstName LastName
1 Jim Smith
2 Nancy Orr
3 Ken Stephenson

And the structure of the classes table is

UserID ClassID
1 AAA
1 BBB
2 AAA
3 BBB

What I'd like to do is select the FirstName and LastName the users table who have a UserID in both Classes and Users.

The tricky part of it is that I only want to return the FirstName and LastName of userID who have multiple ClassID in the classes table. (hopefully I'm making sense when I explain this)

In this case I'd like it to return just Jim Smith's name because he's the only userID who has two ClassID entries.

So far I've come up with

select UserId, count(*) as MultipleClasses
from classes
group by UserID
having count(*) > 1;

which outputs the UserID and a colum called MultipleClasses with how many classes each ID returned has.

how would I do something like

Select firtname, lastname
from users
where Userid=

The output from that initial bit finding users with multiple classes.