Select email so you can read it in Message Pane. The recipient email addresses will be shown in the 'TO' or 'Cc' header fields. left click on email address to see drop down options and select 'Add to Address Book' It will get auto added to the 'Personal Address Book'.

If you left click again it will offer 'Edit Contact' You can choose to change the address book if desired. If you click on 'Edit Details', this will allow you to check that First name and Last name is correct and enter a 'Display' name etc. click on 'OK' to save changes.

Repeat for each different email address to add to address book.

Thanks for the detailed information. However, l was hoping you could copy the email addresses without having to put them into the address book first. Then I could paste the group of addresses into a new email. It seems this isn't possible.

You can left click on email address and use the 'Copy email address' option and then paste it into a new Write message TO or Cc or Bcc field. But it would mean doing this for each email address.

If this is a group of people you are often going to send an email as a group, then after adding them to the Personal Address Book, you could access the 'Address Book' and under 'Personal Address Book' create a 'Mailing List'. Then using Ctrl and click select/ highlight those contacts, then left click and hold down on highlighted contacts and drag them over the 'Mailing List' and drop into list. Now select the Mailing List name and you will those contacts in that list. From then onwards you can easily select the mailing list name from the Contacts Sidebar in a Write message and add all in one go.

See helpful info here: - sorry image is of last version but the info is still correct. https://support.mozilla.org/en-US/kb/how-create-and-use-mailing-lists-thunderbird See 'Create a Mailing List' section and use 'Option 2'.

To copy all the addresses in a selection of messages, there is an add-on:

https://addons.thunderbird.net/en-us/thunderbird/addon/copy-address/

The copy-address add-on is the best solution! Makes it easy to copy all the addresses from a received email, and any unwanted can be deleted after pasting. Thanks everyone for your assistance.