Presently, there's no way to compose a new group email based on conversations in a label.
You can however, create a Contact group from an email message, rather than navigating to the Contacts page.
Compose a new message in Gmail and enter the email addresses of all intended recipients in the To field.
Alternatively, Navigate to your label, select a ...
Open the Gmail conversation/message.
At the right of the recipients fields, click the "Show details" dropdown button.
Copy the list of addresses.
Go to the Google / Gmail contacts manager.
If the Google Contacts Preview UI is shown, change to the old Contacts UI.
Create a new group.
Click the "Add to " group name button.
Paste the list of addresses.
The number of a person is the most basic thing that gets shared in WhatsApp, and it might be the only thing you would know about a person.
There are other messengers who work differently, which your public name or username are the basic information that gets shared rather than your phone number. For example, Telegram.
No, from the help article on delegating access to contacts "Your assistant will now be able to access all contacts in your My Contacts group." which implies they will not have access to any other contact groups you may have created.
Analyze the email headers
Email headers determine where a message is sent, and records the
specific path the message follows as it passes through each mail
Go to  to learn more about the how to read email message headers.
Go to  for instructions on finding headers for a message.
Create and populate the contact label
Go to contacts.google.com
Select menu on left and click "Add Label"
Select a contact
Click on the '...' menu on the right, and select the label you created.
Send email to contact label
Once the you have all your contacts, go back to the contact menu and select your label
Hover over the picture or letter symbol ...
There's a better way.
Go to Contacts.
Select New Group:
At the top of the nav, click on the 'Add to' icon:
Then just paste the email addresses that you've copied from the 'Reply To All' in the other answer.
In your compose window, simply start typing the name of the contact group in the Cc field.
When you see the contact group you want to use, just click on it (or use the arrow keys to highlight it then press enter). All of the email addresses for the contacts in that group will be added to the field. Of course, this works for To and Bcc as well.
Please follow these steps to be able see your groups and to add/remove/modify group properties:
1- If you are in Outlook, then click on the down arrow beside the Outlook logo on the top left. Alternatively, go to the People page here and sign in if you aren't already signed in.
2- You should see something along these lines:
Now, your groups and individual ...
At this it's not possible to group contacts on the Gmail side panel. Try the contacts preview web application.
Google Chat / Hangouts
At this time Gmail users can use Google Chat or Hangouts in the sidebar. Neither of them have a way to show groups of contacts, but Hangouts could be used to have group conversations. A group ...