Car pool is still free but you need the pass now. As far as I understand when you sign up to get the pass you need to have $40 in your account for when using the pass as a single rider so it will be able to take your money. Kinda bogus is you use the carpool all the time. Plus there will be a $3 monthly maintenance fee once the 10 goes in to operation later next year.
I will some more research to see if I'm misinformed.
https://www.metroexpresslanes.net/en/home/index.shtml This looks to me that the carpool lanes are underused.