If you have not yet created an agency please follow the steps in Creating an Agency. Once you have, you can create various roles and permissions for end users, (people whose account the agency manages/people that should be able to manage only one account) and send them an invitation to manage one account.
Keep in mind the only owner of the agency can create a new permission set.
Creating an end-user permission set
You can follow these steps to create an end-user role with a specific set of permissions:
Click on the Profile icon in the top right.
Choose the Agencies option.
Select your agency from the dropdown menu.
On the Agency dashboard click to Create Role.
Type in a name for your role.
From the drop-down choose Enduser role type.
Select the permissions or actions you wish the role to be able to perform and click the Add button.
When you are finished editing your role click the Apply button.
Invite an End-User
When you have a Role that you would like to assign to a person that will be able to manage one account you can invite them following these steps:
Switch to the profile you'd like to invite someone to.
Go to the Profile Settings of the account.
Click to Invite User.
Fill in the user's name (it will be used in the invitation email as a greeting).
Fill in the user's email (this email would become their login with which they will be able to manage the current account).
Choose the role which you would like to assign to the person. (the user's permissions will be restricted depending on the permission set of that role.) Note that you can choose only end-user roles!
If you choose the Auto accept invite option, the user will not be able to decline the invitation
Click Send Invitation for the user to be sent an invitation email.