Ok guys, I found a solution that may help you in case you own just 2 or 3 controllers. What we're going to do, assuming you have a Windows 10 PC (we basically need the xbox app through which we will stream our xbox one game), is to emulate the xbox controllers that we don't have. We're going to do that through these programs:
- Xbox 360 controller driver
As lightsup55 pointed out, there's no need for Xbox 360 controller drivers on Windows 10 but I'll leave the link anyway, you never know :P
Now, here's what you need to do:
* Install both Vjoystick and driver for Xbox 360 controller.
* Extract UCR and launch it.
* Click on IOclasses---->Vjoy---->install SCPVbus.
* Time to program buttons. For the purpose of this achievement we don't need to move or shoot through the emulated controller/s but all we need is using them to select a dummy profile so we will only program these buttons:
To program emulted controller number 1:
1. On the window Plugin Selection choose Remapper (Button to Button) and click "Add".
2. Name it whatever you want but to make it simple to recognize I suggest "A 1st controller".
3. Click on Input Button and press whatever key you'd like to assign to Xbox A button. For guide purposes (even if I used the very same button assignment) I choose "Q".
4. Click on Output Button---->Vxbox stick---->1 (as we're mapping buttons for the 1st emulated controller). Click AGAIN on Output Button but this time select "Vxbox Buttons" (#) and select the button you want to assign (in our case, the first one we assign is "A").
Now simply repeat these steps for the other buttons. NOTICE that when you need to assign D-pad buttons you'll have to select "Vxbox D-pad" in step 4 (#) instead of "Vxbox Buttons".
I own 3 controllers so I emulated just one with this configuration:
W and S---->
If you need to emulate a second controller the steps are the same with the only difference that you need to select "Vxbox stick 2" in the output button (and name the buttons "A 2nd controller", "UP 2nd controller" and so on).
Once you have assigned all the needed xbox buttons you will want to check everything is working by going on Control Panel---->Devices and Printers.
You'll see one or more xbox 360 controllers icons depending on how many you are emulating. Right-Click on it and select gaming device settings.
Highlight "Controller (Xbox 360 for Windows)" and click on properties.
Now when pressing the buttons you remapped on your keybord you will see the red circles on the properties window illuminating hence everything is working fine.
(As suggested by x1001x Puppys, you might need to restart your pc if you get issues with your assigned buttons)
Alright, all the mapping and settings part is done (I know, it's boring but better than buying controllers you probably don't need just for one achievement) so now it's time to unlock the achievement.
1. Turn ON your console.
2. Launch Xbox App on your PC and start streaming your console on the computer.
3. Launch Halo MCC, go in custom games and select CTF and Blood Gulch but DON'T start the match yet.
4. Now press the remapped A button of your emulated controller/s on your PC keyboard and select (with remapped d-pad buttons which in my case were W and S) add guest profile.
5. Once you see all 4 lobby spots full, start the game and the achievement will pop.If you have any questions let me know in the comments. Upvote this solution if it was helpful or in case you didn't like it state why or what can I improve before downvoting.