-a, --append
Add the user to the supplementary group(s). Use only with the -G option.
-G, --groups GROUP1[,GROUP2,...[,GROUPN]]]
A list of supplementary groups which the user is also a member of. Each group is separated from the next by a comma, with no intervening whitespace. The groups are subject to the same restrictions as the group given with the -g option.
If the user is currently a member of a group which is not listed,the user will be removed from the group. This behaviour can be changed via the -a option, which appends the user to the current supplementary group list.
注意
-a和-G要配合使用，否则在把用户添加到新的用户组的同时会从现有用户组中移出。- 当前用户在线的话不会马上生效，需要注销！