Call groups are used when a group of phones or users need to be called at the same time. In Actions/User groups you define which users, phones or numbers are part of a call group. It is possible also to include external numbers.
Call forwards are ignored for calls originating from a call group. Call forward is a user feature, so it can’t apply to phones, and when a user was added to a call group and that user has set a call forward, any call that originated from mentioned call group will not be diverted to the number set as CF for that user.
A users DND status is respected when using call groups from version 4.1.5 and up.
When you add a number to a call group (left bottom window) you can force external numbers to be dialled as well. Do keep in mind that this can lead to unwanted behaviour, think about a voicemail of a mobile phone answering the group call.
It is recommended to always select users for your phone to get the most out of your system.