Adding existing acounts as students to groups
If by group you mean class then no you can't add existing accounts.
Can I add an existing Scratch account to my class? from Teacher Accounts FAQ
Only accounts created by the teacher can be part of a class. However, we are looking into adding this functionality in later versions.
My Profile / OS Projects / My User Page / My Talk Page / My website /
Progress bar to 1000+ Posts - Image not updated lots (Progress last updated: I forget to update this :P)
My browser / operating system: Windows 10 (NT 10.0), Firefox 59.0b13, Flash 28.0 (release 0)
My Scratch Extensions——–If you like this post then give me an internet!——–Sharp Scratch Mod