In order to give access to Customers, Vendors, or Partners, make sure that the required features are enabled so that Access tab becomes available on the forms.
1. Navigate to Setup > Company > Enable Features.
2. Under Web Presence tab > Access section, enable Vendor Access / Partner Access / Customer Access.
3. Click Save.
Note: There are times that after users have provided access to Customers, Vendors and Partners, features are turned-off so that no changes can be applied to entity records anymore. With these features turned off, the Access tab is not be visible even on standard forms, whether the Access tab is set to show on the form or not.