I'm giving the following set of characters a go for the synergies, only just starting though so can't yet report how it works but here's the thoughts:

Protection Spec Paladin
Resto Shaman
Shadow Priest (Draenei)
2x Destruction Warlock

The basic idea behind this is to use an all-caster group to get the most out of the +mana abilities, and also to be able to deliver vast quantities of Shadow damage to drop mobs fairly quickly. A side effect is also a large amount of +sta over the entire party.

The Paladin tanks, and also handily has the Blessing of Kings to give everyone a nice +10% bonus to all stats, this should really help with Life Tap as it gives a bonus to both mana pool and HP. The Paladin's ability to regain mana from being healed by others should also help to keep them topped-up with mana and so be able to continually generate threat.

The Shaman is the healer. Wrath of Air totem should really help with an all-caster group and Mana Tide Totem is available for use when the party needs the extra mana regen. An Earth Shield is also available to make healing the tank that little bit easier.

The Shadow Priest is DPS. Blessing of Fortitude is welcome across the board here, especially on the tank to add survivability and the Warlocks for their lifetap. They should spend their time mostly in Shadow Form to allow them to use Vampiric Embrace and Vampiric Touch to help the rest of the party to regenerate their health and mana. The Draenei Priest ability, Symbol of Hope, also serves to act as another emergency mana regeneration ability for the party.

The two Warlocks are DPS. They should try and make the target as Shadow-vulnerable as they can in order to both deliver maximum damage and let the Priest's Vampiric abilities work to their maximum efficiency. One of the 'locks should have an Imp out at all times in order to provide the party with the +sta bonus, to add to the Blessing of Fortitude and Blessing of Kings.