My goal is for multiple users to be able to sign up and for each user to be able to connect their Shopify account
Hi there @updigitalmg,
Thank you for your message and for getting in touch 
I have carefully checked our range of plugins available and I recommend you use Shopify Custom Apps Plugin.
Note: This plugin is designed to work with Custom Shopify Apps
(It will be possible to use it only if you or your clients have Custom Shopify Apps)
Please check the plugin supporting documentation and live demo page for more information.
Hope this helped, have a nice day 
Best regards