To add a new user to the technical or billing contact list for the organization, you need an existing account to grant you access, as described here.
If the ex employee is the only Technical or Billing contact on the account, and you need additional assistance recovering the account as you have no way of accessing the account locally (i.e. the account was set up via a personal e-mail rather than a corporate account) you can reach out to our customer advocates team at https://support.atlassian.com to help you gain access to the site by providing proof of ownership of the system.
Alternatively if you have domain control of that account, you do everything needed locally, by taking ownership of the ex users account to issue a password reset request to gain access to the account details at my.atlassian.com, then use that account to add in a new contact.
To do this it will vary in steps depending on the local configurations you have set up, and you would need to refer to the system providers documentation for the exact steps, But you would want to reactivate and reset the password for the account on your local domain. Then once that account is re-activated log into that users domain e-mail account and request a password reset from the login page at https://id.atlassian.com/login to send the request to the users e-mail for a recovery option.
Once the password is reset and you are able to get logged in follow the steps in the following KB to transfer ownership to the new contacts:
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