Thank you for the video and other information, no need to apologise.
I can see in your video that the issue is related to the Homepage option you have set. If you go to the User Group option in your CMS and click the button at the end of your Group name A_Group and choose Page Security, you will need to tick the box next to the entry named Icon Dashboard. Save to confirm. You should now find you can add a User type User to that Group.
Here are some screenshots of me doing the same on my own User Group:
Now you should find there is no longer an issue. If the User Group does not have that option ticked that you are trying to set as Homepage, the User will not be able to use it either. The reason it works for Super Admin is because they have all the power and can have whatever Homepage they like, regardless of User Group settings.
Alternatively, you will need to set a Homepage option that is one of the options ticked already in the Page Security options for your A_Group User Group.
Many Thanks.