list all users of a usergroup

[Hornstar]

I was a bit vauge sorry. I meant how would I list all users from a usergroup on a template/page.

eg. I have created a file called users.php and a template called users.

I want to list all users from usergroup x, y z like this on the page users.php:
Usergroup x: user 1, user 2, user 3
usergroup y: user 1, user 2, user 3
usergroup z: user 1, user 2, user 3

My first thinking was there may be a variable?
Is there any variable which can list all users from a usergroup?

My second thinking was I need to query the users in the usergroup, and then display it. (I need help on the code for this part if so)

Thanks in advance.