Players who received a tender but havent signed an NFL contract and unrestricted free agents whose contracts expired can participate in offseason workouts and minicamps with their previous team while retaining the free agency rights they have.
In order to participate, these players must sign an agreement that contains the standard language the NFL and NFLPA came up with last year. It has been incorporated into the 2011 collective bargaining agreement as Appendix Q.
Appendix Q protects players in case they are injured while participating in team activities during the offseason.
Practically speaking, players who are franchised, such as Jairus Byrd and Ryan Clady, and restricted free agents, like Victor Cruz, can choose to participate in team activities without signing an NFL player contract.
A main benefit to signing a participation agreement instead of an NFL contract is that a player will preserve his option of holding out without subjecting himself to penalties.
http://www.nationalfootballpost.com/Participation-is-optional.html