Grab: Standard action to start the grab as long as one hand is free. Minor action to sustain the grab. To move with a grabbed target requires a standard action. Grabbed target is immobilized.
1. Assumption one-handed melee or ranged weapon and off-hand is free (no shield, potion, etc.). That means if I successfully grab and use a minor action to sustain, as long as I don't move I can use a standard action to perform a basic melee/ranged attack or use a power that doesn't move any squares.
2. Grabbed target is immobilized, meaning the target can use the standard action to attack with a basic melee/ranged attack or use a power that doesn't move any squares but not move. The target can continue to beat down on the grabber if desired.
I saw no mention of penalties for attacks. I imagine the common scenario of the hero being grabbed from behind and an enemy punching him in the stomach. Moments later the adventurer uses the cleave power to kick the puncher and cleaves the grabber with an elbow thrust, or vice-versa. I like being creative in the description and not always enforcing the rules to the letter, such as using the body in this case instead of the weapon while still inflicting the same damage.
If helpless allies are considered objects, then why not willing allies. A PC could drag an ally half his movement rate as a standard action. Though I probably wouldn't recommend it as a common strategy, an immobilized ally could be dragged via the grab option while the immobilized ally continues to attack. Of course, now everyone wants to hire a henchman to drag them around the battlefield and hand them potions and other items. Sorry. :)