You can view the ownership details by logging into your dashboard, selecting 'Help and Support' then clicking the 'Account Admin' option on the side menu. From here you can view the owner(s) and key contacts for the account.

To add an additional owner that isn't already a member, you need to invite the additional person to the account. To do this choose Add Users from your dashboard or choose Accounts Settings > Users.  From here you can invite users to the account securely using their email addresses.
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Once you have done this, go back to 'Account Settings' > 'Team Settings' tab on the side menu and click 'Assign Level of Access' on the Owners team. From here you can use the checkbox to select and deselect people in the Owners Team. You need at least one person selected at all times.