Add group with specfic id:
# groupadd -g {gid} {group}

Add existing user to existing group:
# usermod -G {group} {username}

Check to which group a user belong:
# id -nG {username}

Remove existing user to existing groups (group1,group2,group3.group4), e.g remove it from group3:
# usermod -G {group1,group2,group4} {username}