You can add a Trustee using Contacts: click the + button and add a Trustee.
An alternative option is to select You in the bottom navigation bar: select the Trustee panel and follow up on the instructions.
When using Myend to invite a person as your Trustee, they will receive an email with your request. Your Trustee can accept or reject your request. Once your Trustee has accepted your request, it will be updated in your account.