Screening New Members Using the Pending Subscription Notice
NOTE: This option is only available to Restricted groups. See Admin>Settings>Spam Control.
The Pending Subscription Notice is one of the group Member Notices (see https://groups.io/static/help#membernotices). It is sent automatically to anyone who applies for membership in a restricted group. It is not sent to those you invite or direct add.*
The notice marked "active" is the one sent; if no notice is active, none is sent.
Some groups use this notice to ask a series of questions of applicants in support of their application; responses will go to the group owner address and be recorded in the member’s page under "+owner messages."
Send a Pending Member Notice asking prospective members to introduce themselves or to answer your questions. If you wish, you can resend if they don't reply within 2-3 days. If they don't reply to the second probe either or don't meet your criteria, reject their request.
*The logic here is that you have previously vetted them in some other fashion and want them to join, otherwise you wouldn't have invited them. If you need to gather information from every subscriber no matter how they join, you'll have to find some other way to do it.