Let’s discuss the “add users to a o365 group” question in this thread, ok? Just to have all related information in one place.
Regarding the emails, could you please send us screenshots of the workflow and the “send email” action properties, please? What is the error you get when sending the emails? By default, messages are sent via our external mailing service. There may be some issues if different settings were used, so we need to check the configuration.