To add a new owner, first go to the Settings in sidebar menu and select the Owners section.

On the bottom-right you will see the button Add new owner.

Important: you need to be connected with one of the current owners to be able to see this button. Clicking this button would open a new window.
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In this window, enter the name of the new owner and paste the owners address or ENS name.
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Next, you'll be able to set the new signature policy. In this case you can require either one or both of the owners to sign a transaction for it to be executed.

After reviewing again if all change parameters are correct, you can submit the change using the Submit button. Your owner wallet will ask you to confirm the owner change on the blockchain. And will afterwards take you to the transaction details where you can follow the progress of the owner change.

In case you already had multiple required signers for this Safe, the other owners will have to go to the transaction queue and confirm by expanding the transaction details and clicking on the Confirm button.