Unable to add staff/admin

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I am trapped in some sort of loop. I'm the admin of my store.... www.montereyglassworks.com. I am trying to add another admin (the co-owner who also owns the instagram account we wish to link to). When adding the name, it tells me the name is already assigned. When the user signs in, he is not associated with any store. How the heck do I get this user set up as an admin on the account? I'm exhaused...any guidance would be appreciated