Good news! Thanks to cstefanj, you can now create a welcome message for your group, which will be included in the welcome email to newly accepted applicants. You can find the new input field under Edit Group, below the description field. Hopefully this will be a good way to extend a warm welcome (and provide some guidance) to new members!
This is how the email looks like (sorry for the different text, was too lazy to make fitting screenshots!):
Hope you find it useful!