Mary Cooch

You might like to read the documentation on Groups

Do you mean adding students to the site as a whole? Do you want to create student accounts and then have them  be enrolled in  your class? It's worth knowing that in Moodle terms, enrolment is when you add students to a course and authentication is when you add them as new to the site. So students must be authenticated before they can be enrolled. Take a look at the Guide for new administrators and the section on Adding users.

Hello. Yes, sorry; this was a bug and has been reported here: MDL-50515 If you are able to upgrade your Moodle site it should be fixed.

Please see the Backup and restore FAQ which has a question : Restore stops with the message "Trying to restore user xxxx from backup file will cause conflict" that might  give you some pointers.

Look at the setting Enable external blogs in the documentation Blog settings