Kathleen -- This problem with Wordpress is almost an FAQ. Didn't you have this working at one time?
Check out the following:?
/g/GroupManagersForum/topic/67893168#25827?
Please also note that if the prospective member already has a groups.io account and is logged in, the promote form
will not work. I generally recommend that you provide a mailto: link to the subscribe+ address instead, as it is more versatile.
Regards,
Bruce
Check out the groups.io Help Center?and?groups.io Owners Manual