You can create a new Private League when you draft your team. When you start to draft your team, you will be prompted to select a league. One of the options is to create a New Private League. The Private League will have a League Number (e.g. NFL1000) which you will need to give to the people you want to invite.
They will join the Private League in the same manner, except they will select to join an Existing League and enter the League Number. This will allow them to join your Private League.
Please note that Private Leagues are restricted to the $60 million salary cap.
Private Leagues require 24 teams to start. You have 7 days to fill the open spots in your league with your friends. After 7 days, your league will be opened to the public to fill the remaining spots.