Free Agency will be fully open once regular season week one begins. However, until that time, free agency will be restricted.
For the first week of preseason, there'll be no free agency pick ups.
For the second week, there'll be one pick up per team.
For the last preseason week, you can pick up a second free agent.
So by time the regular season starts you can possibly have two free agents and then it's open to all.
