Navigate to the Members tab on the left side of the screen. In the search bar enter the primary member's name, and then click on the corresponding number in the ID column. This will open the membership policy sidebar, then click on view in the top right of the page.

Once the membership policy page is open scroll down to the Additional members section. Search for the desired patient name and click on them in the drop-down.
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You will now be asked to confirm the connection between the new member and the primary member, this will allow the primary member to be set as a billpayer for the new patient.
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You have now successfully add the new patient to the membership, you can easily remove them by finding the policy and clicking remove next to the patients name.