Since you have a Witch, it looks like you have some access to stuff other than the Core Rule Book. The Inquisitor and Alchemist can both do a lot of what the Rogue does best (Skill Points) and might be worth considering.
In general the rogue is considered to be one of the weakest classes in the game. This is because it's primary damage ability "sneak attack" can be incredibly hard to pull off without flanking an opponent. Because the rogue has so few hit points and wears light armor being in a melee position that you can flank from can put the rogue at high damage risk he is often unprepared to deal with, and given the thing he does well "skill points" can be done well by several other classes this makes him less attractive.
This doesn't mean that the rogue is unplayable and he certainly covers skills your party doesn't so I believe he will be a good addition but you want to be careful in how you play him. Try to bump your base attack with combat trick and using a one handed weapon in two hands, take a decent strength and constitution, use magic device should be maxed so you can use wands (especially bulls strength)
Is a human rogue a good addition to the group? I'm not really comfortable (yet) with roleplaying a bard or cleric. Are there any other classes that would help the group out better?
Yeah, try bomb throwing alchemist. It is what I'm retraining my rogue to in RotRL.
LOTS of cramped dungeons where flanking is hard to come by. Then God help you when you need to use a range weapon. Traps have been almost non-existent. All in all rogue was an awful choice for me in the campaign so far.
Is a human rogue a good addition to the group?
A rogue sounds like a great addition to this group. All the other classes have only 2 base skill points per level (though you do have two smarties with the wizard and witch), so you can fill a lot of gaps.
You'll probably want to keep your Use Magic Device high to use things like Scrolls or Wands of Cure Light Wounds - your witch can use them, but if she goes down you'd have a hard time getting her back up (other than with potions).
The Dwarf Fighter will probably be in the thick of things and be a great flanking buddy. If you're fighting lots of little things, you may want to try to flank a different creature than he's attacking, so you don't waste effort - your sneak attacks will take down most small threats very quickly.
You'll almost certainly be the only one able to climb or balance (acrobatics) your way anywhere hard to reach (at least until spells like spider climb and fly come around) - that can be very useful as well.
Enjoy! Rise of the Runelords is a blast!
I suggest a cleric with a two level dip in paladin, good healing and can hold its own thru smiting in combat.
Rogue certainly has some challenges, but a lot of what makes any character either good or bad depends on what you want out of the character. Do you just want someone that will deal a ton of damage and is not worried about much else? Then rogue probably isn't a great choice. Do you want someone that can potentially deal a decent amount of damage (though not always) but is also able to help the party a lot outside of combat? Then rogue becomes a better choice.
Another thought would be to take a look at the Ninja which is technically a rogue variant (though not archetype). I think the ninja gets some better tools to play with in terms of being able to deal damage and help the party. A rogue can take the Ninja tricks and vice-versa, but there are still some things that are unique to both classes.
I also suggest an Inquisitor. I have a half-orc Inquisitor of Erastil and he has been knocking foes into the dirt.
Of course, he has had some very lucky attack roles, but the Class itself is solid and lots of fun. Intimidate is hilarious to RP!
Runelords is a great campaign, BTW.