You need to make sure whomever you are boosting this achievement with must be in the same Darkzone bracket as you are. If you are not sure what bracket you are in, bring up your map, move your cursor over the Darkzone and it will show you at the top right hand corner of your screen. Your Darkzone bracket is determined by your current level (orange experience bar) or your gearscore (you begin to get loot with 'gearscore' after you reach level 30). The first Darkzone bracket is levels 1-14 (orange experience bar). So, if your character is currently between levels 1-14 (orange experience bar), you will only be able to see other players inside the Darkzone who are also between levels 1-14 (orange experience bar). If you are not in the same Darkzone bracket, you will not be able to see each other while inside the Darkzone, even if you are in the same squad and enter through the same door, it won't work. I've tried this personally.
Also, I personally tried transferring non-gearscore weapons and apparel from a brand new character to my primary character (using my stash box) in attempt to lower my gearscore on my primary character. I tried this to see if I could bring myself down to a lower bracket than I originally was so I could boost with someone who was currently in a lower bracket than I was. However, it didn't do what I was hoping, it did bring my character's gearscore way, way down, but it didn't change or lower my character's Darkzone bracket, it stayed exactly the same as it was originally. So, this method doesn't work.
*If you want to boost this with somebody on TA but they're not in the same Darkzone bracket as you, you'll have to start brand new character(s) and play through the "beginning" part of the game. Which, isn't all that bad actually and doesn't take up too much time either, probably like 30-45 mins depending on how fast you play through it.
The "beginning" includes: a small tutorial of basic functions, three easy 'safehouse' side missions, a short 'story' mission, and a few cinematics. After all that, you'll arrive at Camp Hudson. When you arrive at Camp Hudson, bring up your map and set a way point anywhere at the Darkzone and run all the way there. When you arrive at the outside of the Darkzone find a DOOR CHECKPOINT, open it and go inside. This will unlock fast travelling to that location as long as you are outside of the Darkzone (no fast traveling inside the Darkzone). When you enter one of the DOOR CHECKPOINTS, make sure it says something like "DZ01 discovered" in big orange letters at the center of your screen or you didn't unlock fast traveling for that location. You may have to walk to the center of that small room for it to say that. (*If you just run past a DOOR CHECKPOINT without going inside you will not unlock fast traveling for it! Be sure to go inside!)
So, now that you have reached the Darkzone, your character should be somewhere between level 3-5 (orange experience bar), therefore you will be in the first Darkzone bracket: levels 1-14. I'd also recommend discovering more than just one Darkzone DOOR CHECKPOINT only for fast traveling purposes. You will most likely encounter other players inside the Darkzone looking to interfere if they see you boosting Rogue Kills. If this happens, just leave, go all the way outside, invite your booster-buddy back to your squad and fast travel to a different Darkzone door to continue boosting. I'd say only one person needs to get a Darkzone checkpoint in order to start boosting, you could probably just invite the other person to your game after they arrive at Camp Hudson (possibly before then too, no confirmation on that though). If they happen to spawn somewhere else in your game and not where you are, they can fast travel directly to you by selecting your name in 'group managment' and picking 'fast travel to player', just make sure you are completely outside the darkzone so they can fast travel to you.
Hope this helps!!!
