Linux Add user Group

Etc directory there are two files under one passwd a group
passwd GID is the primary group, the other groups are extension groups, and the extension groups are described in/etc/group.
Useradd username if not specified, a GID with the same UID is created by default.
Other groups can be added to other groups by-G when creating users, or they can be modified with usermod-g groupname username for existing users.

For example, the primary group for User1 is 500, and the expansion group is 501 and 502
The primary group of the User2 is 501, and the expansion group is 502
The main group of User3 is 502

The passwd format is as follows:
User1:x:500:500::/home/user1:/bin/bash
User2:x:501:501::/home/user2:/bin/bash
User3:x:502:502::/home/user3:/bin/bash

The group format is as follows:
USER1:X:500:
User2:x:501:user1
User3:x:502:user1,user2

Linux Add user Group