Can I participate in the contest now?
Absolutely!
You can pick any team you want to win their game, but you can only pick each team once throughout the contest. That's why grayruby suggests saving the Celtics until later.
Whoever makes it the longest without picking a losing team wins the grand prize, which is going to be at least 5k sats. I keep adding sats to the prize after each post.
Let me know if you have any questions.