In the Partner Portal you will, as an admin, have an overview of all your clients.
To add a co-worker to the Portal, just head to People and click Add Member in the top right corner. Enter the first name and email address of your co-worker.
A quick way of entering a larger number of members of your practice is copy/pasting first names and emails in from a spreadsheet.
Hit Invite and an email invite is sent to all of them.
To assign your co-worker to a a client, head to Clients on the main menu you will be able to choose which members of your practice have access to which clients.
Once a member has been added to your practice, you can just choose any given client and add the member from the Add Bookkeeper dropdown. When selected as bookkeeper for a company, the client will appear in Clients once the member has logged into the Partner Portal.