There are a few GNU tools for finding out info about the users and groups in your system. You can also grep the users in /etc/passwd and /etc/group to find out the information you need.

How to find out user info with id, groups and finger:
The id command displays the user’s UID and all the user’s groups and GIDs.
Syntax: id username
$ id razvan
uid=1000(razvan)
gid=1000(razvan) groups=1000(razvan),4(adm),
20(dialout),24(cdrom),46(plugdev),
116(lpadmin),118(admin),125(sambashare)
The groups command also displays all the user’s groups:
Syntax: groups username
$ groups razvan
razvan : razvan adm dialout cdrom plugdev lpadmin admin sambashare
If you need to find other info, such as the login shell, full name, email address or homedir, use finger.
Syntax: finger username
You need to install finger first:
- On Debian/Ubuntu/Linux Mint: sudo apt-get install finger
- On Fedora/CentOS: sudo yum install finger
- On openSUSE: sudo zypper install finger
$ finger razvan
Login: razvan Name: razvan
Directory: /home/razvan Shell: /bin/bash
[...]
No mail.
No Plan.
Or, you can grep the username in /etc/passwd and /etc/group:
$ < /etc/passwd grep razvan
razvan:x:1000:1000:razvan,,,elp:/home/razvan:/bin/bash
$ < /etc/group grep razvan
adm:x:4:razvan
dialout:x:20:razvan
[...]
razvan:x:1000: