Did you already make the group? If so you can invite them by right clicking their name in your friends and choose group invite, then pick the group you want them to join. The group must first be made and the people you plan to invite must be in your friends list. If you haven't made the group yet, click community > My…
It's in the friends area. There's an add friend option, where you type in their name. It's dependent on Internet Explorer to function though. So if you have IE setup to reject all cookies, the STEAM stuff won't work.