Groups and Permissions

[gianfizz]

I'm having trouble when creating my groups.
If the group does not have add an change permissions for a specific model, that group can't even access to a view only permission.
If I give the group, view permission only, and log it with the user with that permission, it doesn't work, I can't access to that model and view it.
How can I create the groups with view only permissions and make it work? without it having change and add permissions.