Groups: when users are invited to the group then they should receive normal notification via Notifications module

[AlexTr]

Now only email is sent without using Notifications module.

[LeonidSokushev]

Join procedure might be improved by the same way
https://una.io/page/view-discussion?id=2795

[cbagdatli]

When the users close email notifications -and this is what they do usually- they are having no invitation from groups. This is impacting group module functionality.
İf possible it would be very good for production sites to put this fix to 9.1 update.