global class SandboxRefreshScript implements SandboxPostCopy {
    global void runApexClass(SandboxContext context) {
        // Change value of Alias below to the Alias of user who should own the group, ensuring that it is unique among your users
        List<User> groupOwner = [SELECT Id, Username FROM User WHERE Alias = 'TTayl'];
        Id grpOwnerId = groupOwner[0].Id;

        // Change group privacy, description and name as needed for your purposes
        CollaborationGroup newGroup = new CollaborationGroup(
                                    CollaborationType = 'Public',
                                    Description = 'Group description text',
                                    Name = 'Group for mentions',
                                    OwnerId = grpOwnerId
                                    );

        insert newGroup;
    }
}


/*****   In a Separate Apex Class File: *****/

@isTest
class TestSandboxRefreshScript {

    @isTest
    static void testMySandboxPrep() {

        Test.startTest();

        Test.testSandboxPostCopyScript(
            new SandboxRefreshScript(), UserInfo.getOrganizationId(),
                UserInfo.getOrganizationId(), UserInfo.getOrganizationName());

        Test.stopTest();

        // Change Name value to the name of your group!
        List<CollaborationGroup> chGroup = [SELECT Id, CollaborationType from CollaborationGroup WHERE Name = 'Group for mentions'];
        System.assertEquals(1, chGroup.size(), 'Chatter Group List size not equal to 1');
        System.assertEquals('Public', chGroup[0].CollaborationType, 'Chatter group is not public');
    }
}