After you have setup your site wide payment configuration choices within the PAYMENTS > PAYMENT GATEWAYS admin tool you can configure those payment choices further using the user group specific payment configuration.
Highlights of the group specific payment choice feature:
You would manage these on a user group specific basis within the user group admin tool:
USERS/USER GROUPS > USER GROUP HOME > [EDIT] (next to the user group to configure) > (Scroll to "Group Specific Payment Gateway Settings" section)
The following screenshot should show you where:
To setup a specific configuration for the user group you are editing click the "Use Group-Specific Settings" link. You will click "yes in the "Are you sure?" popup. You would then click the "Edit the Group-Specific Settings" link when back in the user group admin tool and choose the gateways you want to allow users in that user group to use. Don't forget to individually setup these payment gateways just as you did for the site wide configurations as these group specific are different payment configurations that the site wide ones. You can certainly set up both to use the same account configuration but you have the option to use a different one for "like" payment gateways.
To do setup gateways for each user group you would need to do the same for each user group. If you want a specific user group to use the site wide configuration do not set one up for that user group and that user group will use the site wide payment gateway configuration.