
SillyString |

What's a suitable class to join an existing low-level party of Ranger, Rogue, Shaman, and Summoner in an urban setting? (No Occult classes.)
Anything.
There's nothing you NEED to be. Even if its not something usually found in an urban setting.
For variety, be something thats not a ranger rogue shaman or summoner?
You should tell us how you usually play / how familiar you are with the system.
Or, just ask the group if they want x or y.

Blymurkla |

Hm. The party seems to have a decent mix already, but find out what roles they fill currently. Have they had problems filling the front line (perhaps both the ranger and rogue are archers)? Any skills they're lacking?
Swashbuckler, as have been mentioned, would probably be a decent option. As Alderic say, they might not be as tanky as sword and board fighters (but who plays those, two-handed is probably the most common). But with parry and riposte and charmed life, swashbucklers are surprisingly hard to kill. I've heard they start lacking a bit of ommph after level 5, so if you start higher than that you might consider multiclassing. Urban barbarian or urban bloodrager might suit a swashbuckler.
Bards are nice. Combine with cavalier and make a battle herald. Everyone should appreciate those bonuses. Or go Brawler (Exemplar) VMC Cavalier and then battle herald.

Errant_Epoch |

The party you've listed really covers all conventional party bases. The summoner has battlefield control and provides melee allies, the shaman should be able to fix status, heal, buff, etc. the ranger and rogue combined can fight and cover most skills.
So really I think anything would fit. Pick a class that you haven't tried yet, or something you've been waiting on.
Alternatively specialize in divination magic and become the party's fact finder and researcher.

My Self |
Bard, Wizard/Arcanist, and a full BAB noncasting melee martial would all help this party without being too redundant. At this point, you might want to avoid a pet class (Unarchetyped Cavalier, Paladin with a mount, etc.) to avoid cluttering up your battles. I'd avoid Alchemist because of the potential for collateral damage. If you could specify what each party member is specializing in, and perhaps what sort of character (fluff-wise) you want, perhaps we could help more.