Group messages sent via Housebuilder Pro will always display your company logo (selected in Admin > Company Info) in the upper-right corner of your email. This email will not include your normal email signature so if you wish to, you will need to manually sign off this group email.
Also bare in mind that these emails do not support embedded images, meaning that if you want an image included in your manually-created signature, it will not display alongside the text. You can, however, attach up to 20 MB of documents to this group email.