How do I say "who" as in "I have a few friends who are in that class"?
Use '的' to make the second phrase an adjective phrase for the object noun(朋友).


我有一些朋友 (I have some friends)
参加那个班 (attend that class)
我有一些(参加那个班的)朋友 = I have some friends (who attend that class)


More example:


我那个哥哥 (That older brother of mine)
踢足球 (play football)
我(踢足球的)那个哥哥 = (That older brother of mine who plays football)