Can't call a function defined in library folder

[eluser]CroNiX[/eluser]
Are you manually loading Tank_auth somewhere? CI will only automatically load class extensions that start with the MY_ prefix. You probably need to create an __autoload() magic method to do this.

See here.

Also, is the group_id '500' a string or an int? If it's an int your check in is_groups() will always return false because you are using === which also checks the datatype.