Just a follow up on this...
I ended up hiring a developer to create a custom local plugin that is a custom new registration form that ALSO adds the user to a cohort designated in a hash in the URL. I can share that URL as an "unlisted link" to create new users with the desired cohort association.
The biggest drawback to this approach is that I don't have a validation built in. It might be interesting to put both the user's email and desired cohort ID through a hash so that each link is unique and can only be generated by someone with the ability to generate them. How to generate them? I guess with a plugin configuration page? But that might get tedious for a large organization. So maybe a formula that could go into an excel sheet? column 1 is email address, column 2 is cohort, and column 3 is a formula-generated, tokenized link?
Hmmm. intriguing. If anyone has any further thoughts abuot these methods of enrolment, post here or DM me.
-Tay Moss
http://churchx.ca