Thanks for the suggestion. This might work for some cases, but not most of the time.
Most of the time I want to send each user a personalized text specific to them, like I would in an email. Or if I am inviting a bunch at once it will be because they are connected through an event or something like that and I will want to refer to it… but I don’t want to start a new topic in the forum about that. I want to send them an email.
So I guess that is the solution for my use case failing a personalized message upon invite. I will send an email to invite people to the forum and use a spreadsheet to track them as they join. Longhand but tried and true.