BCH (bitcoin cash) has been credited to accounts equal to their balance of BTC at the time of the fork (2017.08.01)

You can cash out your BCH using "my account" -> "send coins" in the website menu. First click "BCH" on that page. You will need to use a CashAddr style address. If your BCH wallet does not support CashAddr then you can use the address converter at https://cashaddr.bitcoincash.org/ - but note: DO NOT put a bitcoin address into this calculator. It is for BCH addresses from BCH wallets that do not support the new CashAddr addresses yet.

What is bitcoin cash?

Bitcoin cash (BCH) is a fork of Bitcoin (BTC). You can read more about it at https://bitcoincash.org/
It shares a transaction history with bitcoin up to the time of the fork. If a wallet had a bitcoin balance at the time of the fork then it is possible to unlock the same amount on the bitcoin cash blockchain. For every BTC prior to the fork there now exists a BCH.

Does this mean Bitminter will support all other forks?

No. Cash out your BTC to a wallet where you control the private keys, then you can take part in the forks you want. Always cash out. Take control of your coins.

Are we mining BCH now?

No. We are currently only mining BTC, plus NMC through merged mining with BTC.

It says my address is not a valid BCH address?

If you are trying to send BCH to a BTC wallet, stop immediately and get a BCH wallet you can send the coins to.

If you are using a BCH wallet that only supports legacy addresses, use the converter at https://cashaddr.bitcoincash.org/ to get a CashAddr type address.

If you are using a multi-currency wallet or online service that supports multiple currencies, make sure you are sending to a BCH address. Use the converter if necessary, but double check that you have picked a BCH address, not a BTC address.