You need to use usermod command. The usermod utility modifies a user’s login definition on the system. It changes the definition of the specified login and makes the appropriate login-related system file and file system changes.

For example adds user didi to the group called pusers.
# usermod -G pusers didi

Both useradd and usermod support following common options:

• -u 1030 : Use UID 101
• -g pusers : Specify user’s primary login group
• -G ftp,admin,www : Specify user’s secondary group membership
• -d /nas/home/user : Specify user’s home directory
• -s /usr/bin/bash : Specify user’s login shell

These are most common options read man page of usremod for rest of all options.

Caution
According to man page of usermod “The system file entries created with this command have a
limit of 512 characters per line. Specifying long arguments to several options may exceed this limit.”