NOTE: You can make someone a primary contact, when creating them as a user or when they already exist.
1. Hover over My Organisation and click Admins & Contacts.
2. If the user does not exist they can be granted access via following the steps within granting a user admin access.
3. If the user exists; click the drop down arrow next to the users name and click edit contact.
5. Click the IS PRIMARY checkbox and then click save.
NOTE: if a user already exists as a primary contact, then this new user will override the current primary user and make the new member the now primary contact for this organisation.