Set up certain roles for them:
https://support.wix.com/en/article/creating-member-roles-6943237
https://support.wix.com/en/article/managing-your-member-roles
https://support.wix.com/en/article/limiting-pages-on-your-site-to-specific-members-member-roles
Wix Users API info:
https://www.wix.com/corvid/reference/wix-users.User.html#getRoles
https://www.wix.com/corvid/reference/wix-users.User.html#UserRole
https://www.wix.com/corvid/reference/wix-users.html#onLogin
https://www.wix.com/corvid/forum/community-discussion/set-member-role-using-wix-code