func getIndex() {

    self.backendless.userService.getPersistentUser()
    let user = self.backendless.userService.currentUser


    var dataStore = backendless.data.of(Groups.ofClass())
    dataStore.findID(
        "B52F6BEA-79F8-A58B-FF15-AF840BCB2A00",
        response: { (result: AnyObject!) -> Void in
            var LookingForGroupJoining = result as! Groups
            // LookingForGroupJoining.UsersInGroup.append(user)
            let index = LookingForGroupJoining.UsersInGroup.indexOf(user)
            print("This is the index number : (index)")
            print("This is the user : (user)")
            print("These are all the values of the UsersInGroup : (LookingForGroupJoining.UsersInGroup)")


            dataStore.save(
                LookingForGroupJoining,
                response: { (result: AnyObject!) -> Void in
                    let GroupJoining = result as! Groups
                    print("Group has been saved: (GroupJoining.groupName)")

                },
                error: { (fault: Fault!) -> Void in
                    print("Server reported an error (1): (fault)")
            })


            print("Group has been found: (LookingForGroupJoining.objectId)")
        },
        error: { (fault: Fault!) -> Void in
            print("Server reported an error (2): (fault)")
    })
}

This is the index number : nil
This is the user : <BackendlessUser> {
    "__meta" = "{"relationRemovalIds":{},"selectedProperties":["__updated__meta","password","created","name","___class","ownerId","updated","objectId","email"],"relatedObjects":{}}";
    created = "2016-03-07 06:06:40 +0000";
    email = "test@test.com";
    lastLogin = "2016-03-11 04:30:01 +0000";
    name = "<null>";
    objectId = "4A955ADD-7991-1AF8-FFEF-3754587B2300";
    ownerId = "<null>";
    updated = "<null>";
    "user-token" = "18C87E0E-2BB3-DB1D-FF63-3CD5FE003200";
}
These are all the values of the UsersInGroup : [<BackendlessUser> {
    "___class" = Users;
    created = "2016-03-07 03:45:05 +0000";
    email = "lekan.adeyeri@gmail.com";
    name = test;
    objectId = "0EB97FF9-FBF1-2E7E-FF30-160BB6CFFC00";
    ownerId = "<null>";
    password = "<null>";
    updated = "2016-03-18 22:33:02 +0000";
}, <BackendlessUser> {
    "___class" = Users;
    created = "2016-03-07 06:06:40 +0000";
    email = "test@test.com";
    name = "<null>";
    objectId = "4A955ADD-7991-1AF8-FFEF-3754587B2300";
    ownerId = "<null>";
    password = "<null>";
    updated = "2016-03-18 22:33:02 +0000";
}, <BackendlessUser> {
    "___class" = Users;
    created = "2016-03-06 04:15:27 +0000";
    email = "tesbhjbj@ttt.tyy";
    name = "<null>";
    objectId = "570CD92B-74EB-1325-FF8F-B866CB6CB400";
    ownerId = "<null>";
    password = "<null>";
    updated = "2016-03-18 22:33:02 +0000";
}, <BackendlessUser> {
    "___class" = Users;
    created = "2016-03-05 04:50:14 +0000";
    email = "test@testmail.test";
    name = test;
    objectId = "A0831585-716E-B027-FF37-539497748400";
    ownerId = "<null>";
    password = "<null>";
    updated = "2016-03-18 22:33:02 +0000";
}, <BackendlessUser> {
    "___class" = Users;
    created = "2016-03-05 14:54:53 +0000";
    email = "test1@testmail.test";
    name = test;
    objectId = "C8A47476-E5D2-B84D-FF5A-2EB605040400";
    ownerId = "<null>";
    password = "<null>";
    updated = "2016-03-18 22:33:02 +0000";
}]