How can I display all users and groups with a command?

You can display with the help of compgen builtin command as follows:

1. To display all users run following command:

   compgen -u
   

2. To display all groups run following command:

   compgen -g
   

However you can also display all users by cut -d ":" -f 1 /etc/passwd.

Here we are going to use getent for the detailed the info

We can list the user with the following command:

getent passwd

We can list the group as follows:

getent group

To fetch detail a specific user

getent passwd lalit

Replace the lalit with your user name. Lalit will not be in every system :)

You can read the more into about getent here