My understanding is there's no way to send an attachment via Google groups web interface.

Alternative would be to use UH webmail web interface.

Log into your UH web mail (http://mail.hawaii.edu) and choose "compose." You should see "Attachment" in the popup. (see below)

Click on the "Attachment" Another window pops open (see below). Click on "Browse" to choose the file. After choosing the file, click on "Add" to add the file to your e-mail.