Might be worth rolling on a few servers just to check their activity, but yeah, AP-US Horde is looking very busy at the moment. Alliance seems to have about half as many people on as Horde, but with all the older players who are coming back to the game, a lot of us will have alts on both factions, as AP-US started as an Alliance F2P server.
If you're looking for help from high level P2P chars Horde side, then I myself rolled shaman, mage and warlock, and levelled them first, specifically to help F2Ps, as well as farming for vial of the sands so I've got a passenger mount. I also try and keep an XP locked 15 on my account, so people can random queue dungeons and just get RFC, to farm satchels.
I may have been away from the game for ages, but on seeing that wargames are now working for BGs, I cancelled my subscriptions for both Wildstar and EVE online, and provided the wargames are still working after WoD goes live, I'll be subbing back to WoW (and quite possibly doing something on the addon, guild and forum/wiki/mumble side of things).
I'm not much into achievements myself, (don't ask me to help you get explorer), but other players have been helping groups farm for things like reputation. Most recently I've seen people helping to farm rep for furbolg medicine pouch, and Felix has been helping F2Ps out on timeless isle.