First of the attribute is "groups" with s not "group".
And you have to give it group ids (xml-ids) instead of simple names.
Througout the OpenERP Code you will find many examples for this, just search for "groups=" in xml files.
some examples for group ids:
- base.group_sale_manager - base module for users with access right "manager" in sale
- base.group_user - base module for basic users
- and so on
if you want already installed/created groups and their ids go into your database and look into the table ir_model_data (filter by model='res.groups'). the column "name" in that table is the xml-id you need.