Membership on WordPress Standalone - how to add additional Membership Admins?

Hi there.

I've recently converted a Multisite to a Standalone WordPress site. No need for MultiSite as there was only one site in the network.

Everything's working great except that my client can no longer see / access the Membership plugin menu. Membership appears to allow only one Membership Admin by default when running WordPress standalone.

How can I add my client as an additional Admin for Membership?

Thank you!