In Thunderbird it's open Address Book and click New List, then do it and name it. The name of the group then appears as an entry in the Address Book.
I believe there's something similar in Outlook but never noticed anything in BT Yahoo! webmail, though I don't use it regularly.
I think in bt mail you go to mail then create a list - naming list - then adding contacts to your named list
You're right imj; it's under Contacts, but just to be different Yahoo! call it a Category.
I am using BtYahoo mail and mine is list - maybe I am still on classic lol
Oo-er! I'm still on Classic and it says Category. Never mind, I use Tbird anyway.
At least the OP will get the correct info from one of us!
The OP may be one of many who prefer classic to the new version. Despite what we have been told, it is possible to revert to classic. Just disable java in firefox, open up mail and it will do it by itself.
There is a detailed thread about it from last year somewhere I think.