Administration Invites are sent out when the account is created. These are sent to the named Support and Provisioning personnel from the Contacts Form provided.
After that, Administrators can add new Admins themselves.
I Have An Issue
There are occasions when the MS filtering can be over aggressive especially with new tenants where no traffic reputation has been built up.
Please inspect your Junk Email folder in Outlook and the Office 365 spam quarantine to see if the message has been blocked:
If the message is not delivered then we recommend sending the invite to another address (e.g. Gmail account) and then clicking the link to log in to Office 365 with the correct tenant user. The address that email is sent has no bearing on the user that is logged -in to accept the link in the email.
If or you are logged in with a different Office 365 user to the tenant you are trying to configure, you should right-click and copy the link in the email and paste this link into a private browsing session ensuring that you are currently NOT logged in to any Microsoft accounts.
If you have not received an invite from the Call2Teams portal you should ask an existing Administrator to send a new invite to an alternative email address.
If there are no Administrators on your account, you will need to request this via a Service Desk case.