default user role 'contributor' on the main blog on Multisite setup and role recognized by

I would like to make the default user role 'contributor' on the main blog on Multisite setup.

I am using the plugin http://wordpress.org/extend/plugins/multisite-user-management/

but I'm trying to set up social network register which doesn't obey the level that I set with that plugin. Initial sign ups become subscribers on register

Thanks in advance
Mark m.