Multiple WordPress installation sharing one user table

I have set up a couple of WordPress sites that all share the same user table, however as outlined in this article: https://hungred.com/how-to/multiple-wordpress-installation-sharing-user-table-touching-wordpress-core/ you get an error about not having permission to view the admin for the other sites because of the table prefix.

The problem is this article is rather old now and the code has changed in the capabilities.php file so I can’t do the hack shown. BUT I don’t want to hack the core code anyway. How can I fix this problem???? Needed asap please. Thanks