Assuming that you are logged into your Office 365 Portal if not then you can login at https://portal.microsofton
You should have a few urls lists, eg: https://domainname.sharepo
Checking ownership of the domain
You can check the ownership of the domain by selecting the domain and clicking the owners dropdown, click on it and choose manage administrators. You will be able to see the primary site collection administrator.
You can add the administrators by selecting the URL and then selecting the Owners button. Once it is selected, you will see the primary site collection administrator and an option to add / remove secondary administrators. You can search for users from the address book who you want to give admin access to or you can also add non-users by choosing option outside of your domain.
This guide doesn’t lists all the features, but i though something will be better then nothing.