Triplebyte is legit.
I went through the process but withdrew before doing the onsites because I did the math about cost of living in California, and it didn't work out for me and my family. (It may well work out for you! Do the math.)
First part of the process is the online quiz, which is timed and takes about a half hour. If you do well here, you'll either move on to a short project, or skip that and move to the two hour interview. (I don't know much about the project thing as I skipped to the interview.)
The two hour interview, all with the same person, is done through Google Hangouts and covers five areas:
They are looking for you to knock it out of the park on any of the first four, and do decently on the others, to move forward from there. You don't need to be super amazing at algorithms to move forward. It was significantly less painful than, say, a Google interview.
If you move forward from there (i.e. you've been accepted into the program), you will be contacted by a talent manager who will suss out what kind of employer you would like, and ask a few questions about your experience. Then they'll match you up with around 10 to 20 employers in the San Francisco / Silicon Valley and/or New York City area. You'll tell them when you're available to talk through Triplebyte's online calendar, and they'll schedule half hour to one hour "getting to know you" calls, where both they and you can ask questions about how the company works, what you're looking for, what they're looking for, and if there's a fit. You can reject employers prior to the call or get more information from your talent manager.
For each employer, after you talk, both you and they can say whether or not to go forward with an onsite. If you both say yes, they'll be on your slate of onsite interviews.
After your calls are done, assuming you have said yes to at least a few companies, you'll then be flown out for onsites with each company. They'll pay for airfare, accommodation, and transportation. They'll also help you with negotiation, if you want.
Overall, I had a great process, even though it didn't work out for me.
If you're interested in trying them out, you can use my referral code and we'll both get $1,500 and a bottle of Dom if you eventually get hired. (They do pay out on this! I never did get my bottle of Dom though... cough)
As for preparation, if you make it to the interview stage with Triplebyte, they'll give you some preparation material. I especially suggest doing the coding project they'll give you; it's not what they'll do on the interview but it's fairly close.
Good luck!