It has now been revealed by looking at specs provided on the Nintendo Switch website that the Joy-Con Grip that is included in the Nintendo Switch box, is in-fact very different to the Joy-Con Charging Grip that is available for $39.95 (placeholder price)
As far as we can tell by the specs provided, you cannot charge the Joy-Cons with the provided grip in the box. It’s in-fact literally a plastic shell that holds the Joy-Cons together. To charge them, you’ll need to either put them back onto the docked Switch tablet (which may be an issue if you’re playing on the TV), or you’ll need to pay $39.95 (placeholder pricer) for the Joy-Con Charging Grip which has a USB-C port.
This is also confirmed in the differing weights. The Joy-Con Grip in the box weighs in at 97g whilst the Charging Grip weighs in at 106g.