There is nothing in Thunderbird called a "group". I'm going to guess this is a Mailing List. Select the message and click Forward. Or use right-click if you need to choose between "as attachment" or "inline". A compose window opens. Add the Mailing List just as if it were a normal email address. Either start typing its name, or pick it from the Contacts Sidebar. Since you're sending a message to multiple correspondents, think about whether or not it's appropriate to show all the addresses. In a business email, we'd normally want every recipient to see all the other addresses. In a private context, it is usually more appropriate to conceal people's personal email addresses, and so you would add their names using Bcc:. Using Bcc: with a Mailing List means that all the member Contacts will be addressed using Bcc:. But you also need to check that your email service honours Bcc: - some don't, and either silently drop the Bcc'd addresses, or convert them to To: or Cc:. So, how do you normally address a message?