This is a fairly easy topic because there are not that many 5v5 combos for ret paladins but they do exist.

First off ret paladins are better off sticking to the 3v3 bracket as it is by far their best one.  Find a warrior and either a resto shaman or resto druid and go to town.  There isn't a lot to these groups, just pound on stuff and if you play with a druid and a healer has to trinket cyclone look for a hammer/repetance followed by a warrior fear. 

Anyway getting off topic the discussion was supposed to be focused on ret paladins roles in 5v5.  Now I firmly believe paladins best spec in 5's is holy but if you want to be a dps class you hvae that option.  I have not seen high level teams utilizing ret paladins but that doesn't mean they aren't viable.

First makeup is the end of season 3 flavor team "cleave" find yourself 2 healers and 3 melee dps (ret paladin can be one of them) queue up and zerg down clothies.  Priests hate you and warlocks hate this combo because it turns us into a pinata.  More complex strats include peeling the rogue (assuming you have one) and having do damage to a mage/priest further straining the other teams healing as 2 targets have MS on them and are taking damage or having the melee jump off the lock for a quick switch to the locks pet. 

Second makeup that can work is actually interesting and a group was going to run it on BG6 but never got it off the ground.  You pair a druid and shaman as your healers (normally this doesn't work because there is no dispel) but when you throw in the ret paladin who isn't using every global anyway he becomes your dispeller, shaman purges and the druid handles the bulk of the healing.  Throw in a warrior and 2 guys have windfury then any melee dps and its basically a cleave team or for a little more skill based comp put in a mage.  Mage can help CC and throw in burst.  Warlock possibly fits in with this group as well.

Those are your two most viable groups as a ret paladin.  I'm sure you can mix in a few more but these dont' suffer any glaring deficencies.  Sorry about the delayed response but i was busy with work and actually playing a few 5's of our own =)