I am running 5 warlocks at the moment with demo spec. So far instancing is trivial with 5 voidwalkers tanking. They are lvl 49 at the moment.

If you run a single warlock in a mixed group then go either affliction or destruction. Spec demonology up to soul link with improved voidwalker, so your pet can offtank when needed.

Then it depends on your total group dps. If you have low dps and things die slow, then go affliction. Otherwise go destruction because affliction needs some time to build dps. On long fights practically all warlock builds are the same DPS wise.

Pros with affliction: You are very unlikely to get aggro because your DPS builds slow but steady. If you get aggro you can just drain life and survive in most cases.
Cons with affliction: if the group has high dps by the time you lay all your dots on a target it will be dead and your dps will be low on trash.

Pros with destro: you are just a shadow + fire mage with pet and have lots of dd potential. If you have mages in group, it's a good combo.
Cons: if you draw aggro and you have no soul link you are more like dead. There is huge aggro chance because your main nuke (shadow bolt) hits really hard at low levels.

Pros with demo: your voidwalker can offtank and your survivability is good... more or less comparable to a mail armor class. Excelent choice if your tank tends to lose aggro or you need offtanking.
Cons with demo: lower DPS at lower levels until you get the big demo talents or spec for extra DPS in other three.