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