Below you can see what is would look like. You will also notice some other features that MixMax adds to the Inbox compose screen.
You can install MixMax right here.
UPDATE 9/13/15: Apparently the contacts team has made some changes and removed the email icon from the groups page. to overcome this missing icon, select the contact group you want to email, choose one contact, then at the top where you see 1 select, choose select all. Next you can click on the three dots at the top right and choose email.
As all of you are aware, Inbox by Gmail currently does not support group emailing. Well, I am here again to provide you all with a really great workaround to accomplish this. This workaround actually uses Gmail to send the message but you don't have to switch to Gmail to perform this task.
Go to inbox.google.com. Click on the Contacts link on the left. Once the contacts tab opens, go to Groups, and click on the group you want to email. The screen on the right will list all of the members of that group. At the top you will see a black envelope. Click that to email the group. A Gmail compose window will appear with all of the members populated in the To: field. Here you can draft and send your email to the entire group. Once you are done, you can go back to the Inbox tab and you will see your email in the Sent label on the left.
And there you have it, another simple workaround to make Inbox by Gmail work for you.