Sometimes when emailing a group from the contacts application in OS X El Capitan one or more of the addresses will be missing from the recipient list in the mail application. The missing recipients can be added manually to the recipient list but they will be silently dropped when the mail application expands the group into its members.
It is very unclear why this happens but a workaround seems to be to delete the mail addresses from the failing recipient and add them again. After that, the group expands to the full list of members.
