This article distinguishes the meaning between an end-user (the person whose account the agency manages) or a co-worker (someone who works at the agency). In general someone invited as an end-user can manage only one account and someone invited as a co-worker can manage all of the accounts in the company(s) under the agency.
Here we outline various scenarios and what steps must be taken in order to achieve the desired outcome. It is assumed that you have already created an agency and connected it to the company(s) of your choice.
Agency Managing an Account for Customer
If an agency is responsible for managing its customer's account, end-user invitation is needed in order to give the clients access to their accounts. The owner of the agency can set an end-user permission set with the permissions the client should have over the account they are invited to (usually only 'view' permissions). Then the owner of the agency should switch to the account of the client and send an invitation with the already created role. The End-User Roles and Permissions can be set up here.
Agency is Managing a Company, but People Should Not See Each Other's Profiles
If an agency has multiple profiles in a company but they wish to have each client only see their own dashboard it is necessary send an invite for every account to the person that should manage it so if you for example have 3 accounts in the company with 3 clients that should be able to manage only their respective account then an end-user invitation is needed for each account. You can set up multiple permission sets and assign a different one for each client or share one permission set for the multiple invitations. The End-User Roles and Permissions can be set up here.
Agency is Managing a Company, but People Can See Each Other's Profiles
If an agency has multiple profiles in a company and is looking maybe for a co-worker to be able to help them manage the accounts or wants to give the clients access to manage all of the accounts in the company, then a co-worker invitation is needed. After desired co-worker permissions are created a co-worker invitation can be sent to the person that should be able to manage all of the accounts in the company(s) under the agency. The Co-Worker Roles and Permissions can be set up here.