I just use /follow nameofleader and /assist nameofleader. Like /follow jinkobi and /assist jinkobi. Has always worked fine for me... If there was some specific mob I need CC'd for extended periods, I'd just write a macro for the character CC'ing and put it on a separate key from all others.

If I was in a raid needing to focus on a offtank target- I'd just /assist offtanksnamehere