# tested on:
uname -a
Linux debian 3.16.0-4-686-pae #1 SMP Debian 3.16.39-1+deb8u2 (2017-03-07) i686 GNU/Linux

# You may need to re-login to active changes 
usermod -a -G GROUPNAME USERNAME;

# alternative
adduser  group; # add username to the group
admin