If you are using Mailgun you can "import" all your users into an internal mailing list and then when you send an email to that list, it will send it to everyone - it's really easy and has some security stuff built in to prevent anyone else form using the list.
We'll definitely have an "inviter" type module - just not something we've done yet.
Hope this helps!
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Brian Johnson
Founder and Lead Developer - Jamroom
https://www.jamroom.net