I am not sure that this will help anyone here.
I am still using OS 10.8. The problem of sending group emails by placing the group's name in the To box has been a long one. What I do is put the name of the group in the To box then I right-click and select "Expand Group." This places all the names of the people in the group in that box. Then, you can send your email without a problem.
What I often also do is send the email to myself (my name only in the To box) and put the group name in the Bcc box. That works without having to expand the group name.
It is probably good practice to Bcc to a group rather than put its name in the To box, thus protecting the names of the people in your group. Doing this also reduces the annoyance on the recipients part of receiving emails with many many names in it. If I plan to forward such an email that has many names and email addresses in it, I usually delete all the names in To section of the text, again to protect the identity of people in the group.
Just my thoughts.