In the labs
update !

The feature is called “suggest more recipients” and offers contacts who would also be interested in this letter. Assumptions it builds on how often and to whom you sent out mass letters in the past and is activated as soon as at least two recipients are entered.
For example, if you often have to send a letter to employees — the boss, manager, techie, supply manager, cleaner :) and you added the manager and manager when writing the letter, then the feature will suggest adding techie, supply manager and cleaner.
As always, to activate - go to Labs.
via
TechcrunchPS I only had difficulty with the ability to offer me 2 more recipients (on the 3rd attempt), although I sometimes send group letters.