New validators can be added at any time. They will follow the New Validator Process flow to start, and when a current validator is ready, they can propose a new ballot to add the validator.
This process is detailed in the New Validator Process Flow - Ballot Process
Validators may be removed through the voting process for various reasons, including failure to maintain or upgrade a node, malicious behavior, or self-removal. Any current validator can initiate the process to remove another validator.
1) Go to https://voting.poa.network/poa-dapps-voting and select xDai Stable Chain from the network dropdown. Check that your web3 wallet (Metamask or Nifty Wallet) is connected to your Voting Key on the xDai Chain.
2) Click on the New Ballot Button. The Validator Management ballot will appear by default. Fill in the required values.
Description of the ballot: Add specific rationale for removal along with links to any pertinent information (ie forum posts where this issue has been discussed).
Remove Key: Select this option.
Mining Key: Select this option. Note: When the mining key is removed, associated payout and voting keys are also removed, so you only need to select the mining key here.
Affected Key: Enter the mining key of the validator to remove.
Mining Key: Select the same mining key from the dropdown.
Ballot End: The date will pre-populate with the earliest available end time. You can increase the duration if desired.
Add Ballot: Press to confirm and sign the transaction in your Web3 Wallet.
3) The Ballot will be displayed in the All and Active Ballots sections along with the required number of Yes votes needed to pass (and remove the validator) and the ballot end time.
4) Inform current validators about the new ballot (through the Slack Channel or other means) so that they have a chance to vote before the ballot expires.
5) Once the ballot expires, any current validator can finalize it. Connect to the xDai Chain with your Voting Key and click Finalize to complete the process. If enough votes were collected, the validator will be removed from the validator set.