The causes of these kind of problems is usually a combination of permissions and group settings.
By default
PN ships with two groups admin and users. If you change the name of the users group then you need to update the permissions to reflect this change of group name.
You also need to change the default start group for a user under administration->settings.
Failiure to do both of these will result in user falling through the permissions system with no rights to the content.
These earlier threads cover this issue
http://forums.postnuke.com/index.php?name=
PNphpBB2 [40]&file=viewtopic&t=8139&highlight=default+group
http://forums.postnuke.com/index.php?name=
PNphpBB2 [41]&file=viewtopic&t=8710&highlight=default+group
http://forums.postnuke.com/index.php?name=
PNphpBB2 [42]&file=viewtopic&t=9204&highlight=default+group
-Mark