New user / site permissions presets

Hello,

I have a mu site and using marketpress / framemarket / gridmarket and I have a problem. When a new user signs up for just a user name or a site they are granted "Subscriber" permissions. I want them to be able to add items or delete their items and subscriber permissions does not allow this. How do I set the default permissions higher, and what should it be set to to only allow them to add items, and edit/delete their own items?

Thanks!