How to change give "Author" permissions to a mailbox in Exchange 2010

Here is my issue:

We have a shared mailbox that two departments use.  let's call it "Inventory Request Mailbox".  I've created a outlook form that the users can fill out and then send.  This form is coded to send to the "Inventory Request Mailbox" where the 2 departments can see.

Now here's where it gets tricky.

Originally in exchange 2003, i was able to give group A access to the mailbox and group A would be able to only modify their own email/form within that mailbox.

And group B would have full access to the forms in that mailbox

With Exchange 2010, I can't seem to duplicate that.  It seems that its Full access or just read.  Even if i modify the permissions in the mailbox level, it doesn't work.

I'm thinking that this can only be done via powershell.   If so, can someone please explain how.

Thanks