So groups.io has the basic invitation:
You have been invited by [so-and-so] to join the Groups.io group [group].
If you have questions about this invitation, send them to [owner email].
To accept the invitation, please reply to this email, or you may accept the invitation on the website.
If you are not interested, or if [Invitee Email Address] is not your email address, please ignore this email.
The Groups.io Team
I would like to use the [Name] and [Invitee Email Address] in a custom welcome message. I tried sending an invite using the hard brackets but it was no bueno.