First you need to create three groups (Admin, Moderator and Users) like you mentioned - here is the example for Admin group.
$group = Sentry::createGroup(array(
'name' => 'Admin',
'permissions' => array(
'admin' => 1,
'create' => 1,
'delete' => 1
),
));
For other groups, you can make changes in permissions
array. You can get group permissions with -
$groupPermissions = $group->getPermissions();
There you go!
To assign users to specific groups, you can just added like below.
$adminGroup = Sentry::findGroupById(1);
$user->addGroup($adminGroup);
All you need to do is to select any (created) group with dropdown to assign on user create. Or, you can simply assign user to the default group.
Anyway, You can find the Sentry documentation in Cartalyst/Sentry
package inside vendor.