Hi Carlos,
Don't make the mistake I seem to always do...
I'll add new groups -- and forget to add them to my user profile as groups that I can then assign reports to! -- then in report design -- those groups fail to show up in the list of available user groups -- and I'll scratch my head -- until I remember! Details, details....
Just because you created the group and are an administrator doesn't mean you automatically become a member of that group -- or that it is one of the groups you can assign to. (Two separate things!)
Regards,
Bob