Inviting your friends to your Fireteam is simple. Once you are in a Fireteam lobby, click the “Invite Friends” button.
Here you will see a list of your Steam friends. You can organize this list by Name or by Online Status.
In this menu, click on the + icon left of a name to invite someone to your Fireteam!
You can add Steam friends to your friends' list by clicking the “Add a friend” option under the Friends tab on your Steam browser.