I'm not especially familiar with Pathfinder's classes, but just for the sake of making a recommendation other than bard I'll suggest an Oracle. It's a charisma-based class which, though not as skill-oriented as the bard, may make a half-decent face character. Taking the legalistic curse will give you bonuses to some social skills starting at level 5, plus to the ability to make a promise once per day to gain a +4 morale bonus to a roll starting from level 1.
In addition, it has full 9-level spell progression. The druid and witch are both capable of casting healing spells, but both are prepared casters and neither are able to cast healing spells spontaneously - plus, we know their classes, but we don't know whether either of them is planning on healing at all. Considering the size of your group, having a couple of Cure spells on your list will take a lot less pressure off of them to keep the rest of the party standing.
A bard could serve the same purpose, but not nearly as well considering their 6-level spell progression.