Prominence Poker

This can be boosted easily with alts or other players in head-to-head. You'll need a fair amount of chips to make this easy, but regular daily cuts will carry you through, and you can move chips around privately.

During the preliminary stage, you start at a silver rating (even though you can't see actual ratings). With 4 or 5 wins, you will come out of preliminary at a gold rating. During this stage, you will likely be paired against other preliminary players at the same level.

While in preliminary, you can tell what tier you're in by the buy-in price.
5k = bronze
10k = silver
20k = gold (You won't be higher in preliminary.)

If you're just going for gold, stop after 5 games. Again, if you won 4 or 5 of these, you should already be gold.

If you're going higher, read on:

The important part to higher tiers is to make sure you never lose with your main account, so each boosting partner should make several alts to push your mains up. Ideally you need one alt that is gold. (Note: if you lose, it isn't the end of the world as it might be in some other games. After 1-2 wins, you should be back to where you were.)

When searching, let one person search alone. If it immediately puts you into a game, quit out. If it shows you searching, the second person should then search. You should simultaneously see the game starting screen. During peak hours, you may have problems pairing against boosting partners... the best times, I've found, are around 3-6 AM Eastern US time during the week. (Yeah, super convenient.)

A key is that every alt you use is automatically silver. You can use new alts to push one account to gold. Once your main and an alt are gold, have your mains win against each others' alts. When an alt falls to silver, bump it back up with another alt. Repeat until the mains are platinum. It should take around 4-5 straight wins per tier.

Seasons last 2 months, and you only go up or down when you play. As long as you have platinum at the end of the season you'll earn this, so if you're specifically going for platinum, stop playing ranked once you're there. It should pop when you log in at the beginning of the next season.

If you're looking to go to diamond, it's currently not terribly hard. As it works now, you don't need platinum alts. You can use gold. Have your platinum main start searching. If there are no other platinums searching, after around a minute or so it will start searching gold. Have your gold start searching and... yep. Again, about 4-5 games to diamond.

If you can match up regularly with a partner, you can likely both get platinum in a few hours, or diamond in the course of 2-3 boosting sessions of a few hours each.