To give a Team Member access to manage one of your Clients, please go to your Clients section and click on “Edit”  link for a Client.

For Card View :

For List view:

Then select the Team Member you would like to have access to that Client and close the pop-up.
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Once done, your Team Member should see the Client section in their account and should be able to manage this Client. 

​This article should also be helpful: 

How to use the Client Management portal?