OffTopic is a very simple web widget generator for mailing lists.
Subscribe (n.b. do not "invite", just "subscribe". OffTopic do not sends confirmation mails) the address firstname.lastname@example.org to your mailing list, and in your website include an iframe such as
The mailing list actually subscribed to OffTopic are:
This free service is implemented and operated by Roberto Guido. Code is available under the AGPLv3 license. Read here for the full story behind.