Instructions
Place your mouse cursor over your Steam name at the top of the Steam window and click "Groups" from the drop-down menu.
Click the "Create a New Group" link to begin the process of creating your community group.
Fill out the "Create a Group" form, entering such information as your group name and group URL, and click "Create" when finished.
Click "Create" to create your Steam group and navigate to the group by click "Groups" from your user name drop-down menu.
Click "Group Chat" to the right of your created group in the Groups list to open the chat room.
Click the arrow to the right of your friend's name in your Friends list, hover your mouse over "Invite to Chat Room" and click the name of your group to invite that friend to the room.