In previous releases 1.2.x I can add users to additional groups. But now this feature disappeared.
[edit]
vyos@vyos# set system login user test
Possible completions:
> authentication
Password authentication
full-name Full name of the user (use quotes for names with spaces)
home-directory
Home directory
[edit]
vyos@vyos# set system login user test group floppy
Configuration path: system login user test [group] is not valid
Set failed