From your BigCommerce dashboard, go to Account Settings > Users
Find the user you would like to grant access to, click the ellipses in the Action column for their user row and click Edit.
Scroll down to Single Click Apps, check the box next to PartFinder. Then click Save.
That user will now see the PartFinder app in the My Apps section of the Dashboard when logged in.
Note: if you uninstalled and reinstalled the app, you will need to re-authorize additional users.