Need one more neutral bus connection

Hello-

I posted this in alt.home.repair but I thought I'd ask in a more specific newsgroup as well..

I'm adding two double pole 20A 12-3 wire circuits to my breaker box for

my basement. Each of the two 12-3 runs is split into two 110v circuits

(alternating circuits between outlets along the run) with the neutral for each run being shared between the two alternating circuits.

Because I have two of these runs, I have to connect two white neutral wires to the neutral bar in my breaker box. Unfortulately, I only have

one open connection on my neutral bar left.

**Since this is my main panel for my house**** (not a sub-panel). What do I have to do so I can get the extra neutral connection I need? I have open neutral bar connections. Because this is my main panel for the house, can I use one of the open connections on the ground bar for the final neutral wire I have

And before anyone asks, no my 150AMP Siemens panel is not overloaded.

Thanks, Kevin