As an example we will show the steps to enable SourceCode editing for Administrators only:

First of all we have to disable SourceCode plugin in RichText Editor Settings page, if checked the SourceCode Editor Plugin is enabled for all users and there is no posibility to manage the advanced permissions.

If checked out the Source Code Plugin permissions will become active under the advanced Editor permissions visit: http://yoursite.tld/index.php/dashboard/system/basics/advanced_editor 
Note: change yoursite.tld with the URL of your Website

Now we can manage the permissions, in our case we have two groups "Administrators" & "Manager".

Click on 'Source Dialog' link to open the permissions-popup and choose the appropriate groups.

 Enable it for "Administrators"

As we see below "Source" button is available for Administrator but not for manager: