How can i participate already?
Mod-Edit: for CLI examples please look at the updated post below
The tooling is already there, you can use cardano-signer to generate the needed registration-metadata.json file.
The registration on MainNet is live, but please if you wanna do some testing, help us by registering your Calidus Pool-Key on Preview-Testnet for your Preview-Stakepool too!
SPOs on the Preview-Testnet, please help us to test out this new key and submit your registration metadata for your PreView pool on the tesnet. 
You only need two commands to generate the registration metadata json file:
Generate your new Calidus Key-Pair
cardano-signer keygen --path calidus \
--out-skey my.calidus.skey \
--out-vkey my.calidus.vkey \
--json-extended
Remember the given Mnemonics in the JSON for later import in a LightWallet of your choice.
Generate the registration metadata signed by your pool key
cardano-signer sign --cip88 \
--calidus-public-key my.calidus.vkey \
--secret-key my.pool.skey \
--json \
--out-file calidus-registration-metadata.json
You can than use this metadata.json file in a transaction on the cli or simply copy it into eternl-wallet as a custom metadata and submit it in a transaction on the preview testnet.
More details on the generation can be found here.
Registration on the Preview-Testnet can already be checked via the koios api:
curl -s "https://preview.koios.rest/api/v1/pool_calidus_keys"
you can use filtering like
curl -s "https://preview.koios.rest/api/v1/pool_calidus_keys?pool_status=eq.registered"
or
curl -s "https://preview.koios.rest/api/v1/pool_calidus_keys?pool_id_bech32=eq.pool1w83ux7vrwafrsuz044knxl9xxtr38q5jj2l3p28fl3g26faqqwg"
Feel free to ask questions in this thread.
Thank you very much!
Best regards, Martin