ZOMG 10 Warlocks in WSG would OWN

As for granting levels, I don't know exactly how your accounts are set up, but if you level 5 warlocks, say A B C D E, then B and C can gift the levels to new toon F, and D and E can gift to new toon G. Now, F and G can gift their levels as well to a new toon H and I. You just need to level H and I from 1 to 30.
Now, you have 9 toons ABCDEFGHI which are level 60.
And then, for toon J, just use your pre-existing level 70 warlock.

Of course this would mean that your accounts have to be linked properly :P

If not, then I'd say just doing instances with 2 groups of 5, you get good exp from quests that way. Don't do raids because you can't get credit for non-raid quests while in a Raid group.