Our experience has been that open groups seem to attract the largest amounts of bogus joiners.
And, if you have a group specifically targeted, as we do to a rare disease or similar tight focus the number of bogus entries goes down significantly.
We still weren't sure about some of the applicants so we instituted our own, separate form and process (Request to Join) that asked enough questions to enable us to usually just enter name, address into Google search and find matches. Over time we've added questions that give us even more information and it doesn't seem to slow down Requests at all. In fact, since coming over to groups.io our monthly requests have increased significantly but that could also be a result of our greatly increased use of our Google for Nonprofit Adwords account that is running at almost the monthly maximum value allowed.
Anyone who tries to join directly receives an automatic email telling them how and why they need to access the Request to Join form. We don't take any further attempt to access them. From there on it's up to them.
It must be ok because in the last 2 months we've received almost 150 Request forms and direct invited all of them while about a dozen never filled out the form.
We feel that if they are serious about joining, the form is not a deterrent. After all, most of us have our entire personal profile available on the internet with very little effort required to access.
If your group has a serious purpose you owe it to your members to ensure that newbies are reasonably vetted.
Spammers occasionally fill out one or other of the forms on our website but typically reveal their falseness in some quirky remark or putting in gibberish because they get bored with the form
The Corneal Dystrophy Foundation
Fuchs Friends & Fuchs Friends UK groups