Hi, I have already asked this to the SEER Central Team. He said that if you want to join the witness there is a technical document for it, but currently it is in Chinese.

I will give a little explanation with anyone who can join the witness. So, there are 101 people who can join the witness and of course you have to need enough funds.
The first 21 witnesses can get : block produce rewards+collateral rewards.
The 22 to 101 witneees can get : collateral rewards.

If you want to join the first 21, then your collateral amount should be more than the 21st witness's amount and if you want to join the standby witness, your collateral amount should be more than the 101st witness's collateral amount.

You can check all witnesses in this link : https://wallet.seer.best/explorer/witnesses