This is pretty easy to do. Anyone that has played RBG in season 9 at a high rating and hasnt lost a ton of games since then, should have pretty high mmr still. The FC of the RBG group I played with some months ago had 2900 mmr still, from s9, even though he had lost alot of games in s10 and s11. He played at 2600-2700 in s9.
Why this is possible to do is because of blizzard not having fixed the fact that if 1 player joins the team your fighting, that 1 players individual mmr will be the team your fighting's mmr. So if he has 4k individual mmr, you will fight a 1 player 4k mmr team. If you have 2 characters with 4k mmr, you can que up against eachother, only join with the two 4k mmr players, and play 4k versus 4k. This was also the case in arena before, but it was changed. In arena, even if one player joins alone, the 1 player team will have the entire que'd groups mmr.
To que up to RBG, you need 10 players in both teams.
How many accounts you need depends on how many people you are boosting every game. You always need 20 people. Not all of these accounts will join the game. Most of these accounts, you are using to be able to que up. The players that will join the game are the players with the high mmr.
Have you ever opened your scoreboard while in RBG, and seen the MMR change as more people join? Have you seen how it goes up and down every time a player joins? If 1 player joins, the scoreboard will show that one players mmr, if another joins, the scoreboard shows 2 players combined mmr, and so on. If my individual mmr is 4k and I join alone the enemy team will be playing against 4k mmr.
So you need 2-3 accounts with high mmr. The losing team will have 1 high mmr player. The boosting team will have 1-2 high mmr players, depending on how many characters you are boosting. The more players getting boosted, the more high mmr characters you need. The more high mmr characters you have, the more people you can take each game, and the more money you earn.