When a new partner is starting to manage in CP, and they are not able to see all the accounts from their customers in order to manage them, and the partner is just able to see his personal account, a few things should have to be checked before.
1: First thing is check if your admin account is already located in TOP of the sctructure, for example, this a new partner called demopartner_garcia and he needs to be able to manage his customers (abft-ag.ca, catalonia.cat) attached under the partner account.
2: The partner account (firstname.lastname@example.org) should to be added into Service Dashboard->Role and management and contacts, be aware that just the Admin role will have whole rights to manage everyting, roles like director doesn't give the enought rights to manage users and emails.
3: In case Partner will give some admin rights with someone on his team that just will have rights for a customer, and not for all customers, for example for abft-ag.ca customer domain. Then select the domain customer, and add the user just on this domain Service Dashboard, in order to manage it.
4: Sometimes partner is not able to see all his clients even if he is an Admin, just sees his personal account because he is not in the TOP of the structure, He should go to the top of right corner and deploy the tree, then select which client will manage and so he can select it and will go into this customer environment.