I don't think you can do that from within your family tree website. The way you can maybe do it is create a Group in the address book of whatever email service you use, then add each member of your family tree into that group - you can copy-paste their addresses from the family tree into the email Group - that step takes some time. Then if you get a new family tree member join in you just add that one person into the Group. I have a Group like that and I send the email to the Group, which then sends that email to each person in that Group. Hope that might help.