Figured it out. Using default Google contacts app, select the davx5 account, then create a label. You can then add the label to members of the group.

To send an email, start in the contacts app, open the label, hamburger menu > send email.

I cannot get the mail client #K9 to autocomplete and expand the label, which is a drag, but it's better than trying to add (and remember) the 10 people in the group.

I'm assuming the label won't sync back to Fruux or Posteo as a group...