I am sad to see people who like to team up with low Elo friends being lumped together with smurfs, even to the point where some people are suggesting to ban people from playing together if their ratings are very different. Without smurfs, the problems are really not that big. I will explain why.
Suppose that your true team rating is 2000 Elo and your friends true team Elo rating is 1000 Elo, and you start to always play together. Then your ratings will get an initial bump, but very quickly you will be matched against teams who you have a 50% winrate again, and the games become balanced again. There is no harm done here. The same principle applies if you have multiple friends, or donors, whos Elo is way lower than yours, and you alternate playing between them.
Now suppose instead that you sometimes, but not always, team up with your low Elo friend; let’s say only in 50% of your matches. Without your low Elo friend, your rating would be 2000, with him it would maybe be 2200, but now you are at 2100. This means that games with your friend are too easy (and your opponents may accuse you of smurfing) while games without your friend are too difficult to win. This problem was way larger with the previous method of calculating rating change, but it is undeniably still there. As @gnarfk has pointed out, an easy fix is just to slightly change the way teams are matched against each other if the rating between teammates has large differences. For example, maybe a team of a 2000 and 1000 rated player should be matched up against 1600s or 1700s instead of against 1500s. Last year already I suggested a way to ensure this happens (this is the post @gnarfk is referring to):
Now suppose that you do play with a 1000 rated friend and you and your friend win more than 50% of your matches and thus gain rating; but before you are at the point where you start getting balanced games, your friend created a new smurf account at 1000 rating and you repeat the process. This is how players, even with a high rating, can get absurdly high win rates, and this is the main way smurfs can abuse the team game ladder right now. Something obviously needs to be done about smurfs. My suggestion to change the way teams are matched against each other would make it way harder for smurfs to operate in this way, since with my calculation, the lower rated you are compared to your teammates, the smaller the effect you have on determining what other team your team will be matched with. Still, it does not eliminate smurfing completely.