Thanks Liz Eipe for your solution, but unfortunately that did not work for me. What did work for me, was changing the 'capability' for all the add_menu_page() and the add_submenu_page() hooks.
Basically the capability value you set, corresponds to access for different types and combinations of users. You can view all the possible capabilities and their access here.
I choose 'publish_posts' as value, which gives Super Admin, Admin, Editor and Author access to the admin page you register with your hooks.
This was my code before:
add_menu_page(
'My Menu Title',
'custom menu',
'administrator', // Only a admin access
'myplugin/myplugin-admin.php',
'',
plugins_url( 'myplugin/images/icon.png' ),
3
);
And now with Admin, Editor and Author access it's changed to:
add_menu_page(
'My Menu Title',
'custom menu',
'publish_posts', // Admin, Editor, Author access
'myplugin/myplugin-admin.php',
'',
plugins_url( 'myplugin/images/icon.png' ),
3
);
One last small note; do not forget to change this for all the plugin admin pages you want other users to have access to, like all the submenu pages.