Group Permissions

There are two ways to go to the "Group Permissions" page. It doesn't matter which one you choose.

Then

You will be presented with a long list of "Permissions".
If you like you can select the name of the group you want to change from the popup list.
Decide what you want your user to have access to, and (de)select the appropriate box.
If you DO NOT want your User to modify ALL PAGES, do not click "Modify Any Pages". If you want your User to have access to One or more specific pages, see the next paragraph. There you'll find how to add the appropriate permission in that case.
Click submit.
You'll be back at the Group Permissions page, and will see the message 'Permissions have been updated'.

In the top Menu, select "Content/Pages".
Choose the page you want to add/remove "Additional Editors" to and click its link.
When the page opens, you will notice three TABS just below 'Edit Content'. Click the middle one, called "Options".
At the bottom of this Options page, is a field called Owner, usually this is the username of the person who designed the site, and can also be the owner of the website.
In the dropdown list you can change the owner of this page, if you wish.
Just below this Owner's field, is another long skinny field, called 'Additional Editors'
To add a user as an additional editor make sure you first press the control key and then you press the name of that user. If you don't first press the control key, other editor's will be deselected from the list, which is something you probably don't want. With using the control key you can easily (de-)select multiple users from the list.
Click the SUBMIT button.
At the top of the list of Pages you will see, 'The content was successfully updated'.

Remark: On a Mac - hold down Command/Apple key and click once to add/remove highlight

You're done! To test it works, logout of the CMS, then log back in as the new User - and you should only see ONE page for you to access.

User Handbook/Admin Panel/Users and Groups/Group Permissions