The file web.config contains many settings for the website. Changing the wrong thing in this file can have consequences for the entire website and for this reason it is not appropriate to give a lot of people access rights to make changes to this file. Many see being able to change the file as a must if you have a number of editor groups. All editor groups must have access to Edit mode, which is specified in web.config..
Permit all editors to be members of two groups to avoid too many changes in web.config. One group that gives them access to Edit mode and another group for the place in the tree structure that they are authorized to edit.
When you install a website on EPiServer CMS the WebEditors and WebAdmins groups have not been created. You will have to do this first in order to be able to work with these groups. Depending on which providers you have selected for storing groups and users, begin by creating these two groups there. If you have not made any changes to the standard installation, Windows is the default provider. This means that you must start by creating the groups in Windows.