Yes the challenge is still very much active and you can still join. All you have to do is to download the Bitcoin core and run your node till it is fully synchronised. You can click the link to the thread, read the instructions and follow accordingly. Make sure to report on a daily basis so your records can be taken by the OP of the challenge.
Interestig, I actually have a full node, which was shut down last year in the summer, so I only need to fire it up.
I will have a look into the OP ann more closely, thanks!