/cast [nomodifier, target=party1] Dispel Magic
/cast [modifier:alt, target=party2] Dispel Magic
/cast [modifier:ctrl, target=party3] Dispel Magic
/cast [modifier:shift, target=party4] Dispel Magic
It's simple. Bind it on a simple key ( meaning let's say, Q - meaning no shift+Q or w/e ) - and don't have any binds to that key except that one ( so no shift+Q bound for xyz, and alt+Q doing zyx ).
Let's say you've bound it on Q:
Q - dispels first person in your party
alt+Q - dispels second person in your party
ctrl+Q - dispels third person in your party
shift+Q - dipels fourth person in your party
you can replace partyNUMBER with names of your partners if you'd like, meaning you can always press Q to dispel your healer partner or something, although if you swap comps a lot like I do, you'll learn to appreciate doing it this way.
May sound complicated, but in reality it's quite simple and worth getting used to.