Administration -> Configure -> Settings -> Advanced Settings -> Content Access -> Check on "Allow user default access"

The question is what kind of user permissions you are referring to. The setting @RvR pointed to would allow the users to alter the default selection of the access permission option when user post something. Still they could select a suitable access level for each posting individually. The other setting on the advanced settings page (default access permission) would also only change the pre-selected access level (while users would still be able to change the level individually for each post).

Or do you have any other kind of user permissions in mind?