Hi,
the Groups plugin does not change a user’s role. Whatever caused this was either another plugin or someone changed it manually, but certainly not Groups.
Regarding the table you mention, that’s expected and is not involved in user roles.
You can simply change the user role back by logging in with another account that has the Administrator role and editing the corresponding user’s profile, setting its role to Administrator.
If you don’t have such an account, you can set the role of the initial administrator user, whose user id is 1, using a database query – but make a full backup of your site and its database and be sure you know how to restore it before you run any such queries:
UPDATE wp_usermeta SET meta_value = 'a:1:{s:13:"administrator";b:1;}' WHERE user_id = 1 AND meta_key = 'wp_capabilities';
This is assuming that your setup uses the default wp_ database table prefix, which it seems to do based on your report.
Useful documentation is the Roles and Capabilities page.
