Soldier, Medic, and 2 Spies go Offense. If they all have mics and work together it could roll. Spies sap just ahead of the Soldier/Medic and kritzkrieg should clear out the rest.
2 Engies, Spy, and Sniper on Defense. Again, if they communicate and work together they can lock down a base. Spy and Sniper should keep any uber from making it to the Sentry Guns. 2 Engies covering each other should handle anything else. Soldier/Demo won't be able to camp around corners and pick off sentries with that spy around.