You need to select "Send Welcome Email" on Emails Menu
Please, take into account the following:
Salesforce sends welcome emails when:
A community changes from Preview status to Active status. An inactive community doesn’t receive emails unless the associated org has a default portal or if the user’s profile is part of an active portal.
An admin ...
If you want to use letterheads, then you will need to use the Rich text editor by creating an HTML template with Classic letterhead which has the ability to make text bold italics, add an image and then add a link to text or the image. Else, you can create a custom email template which is always without letterhead (This is in classic view).
You can always ...