NFL agents ("Contract Advisors") are governed by NFL Players Association regulations. The following are some of the more relevant provisions.
"(1) Representing any player in individual contract negotiations with any Club unless he/she (i) is an NFLPA Certified Contract Advisor; (ii) has signed the Standard Representation Agreement with such player; and (iii) has filed a copy of the Standard Representation Agreement with the NFLPA along with any other contract(s) or agreement(s) between the player and the Contract Advisor;
(2) Providing or offering money or any other thing of value to any player or prospective player to induce or encourage that player to utilize his/her services;
(3) Providing or offering money or any other thing of value to a member of the player’s or prospective player’s family or any other person for the purpose of inducing or encouraging that person to recommend the services of the Contract Advisor;
(4) Providing materially false or misleading information to any player or prospective player in the context of recruiting the player as a client or in the course of representing that player as his Contract Advisor; ...
(21) (a) Initiating any communication, directly or indirectly, with a player who has entered
into a Standard Representation Agreement with another Contract Advisor and such Standard Representation Agreement is on file with the NFLPA if the communication concerns a matter relating to the:
(i) Player’s current Contract Advisor; (ii) Player’s current Standard Representation Agreement;
(iii) Player’s contract status with any NFL Club(s); or (iv) Services to be provided by prospective Contract Advisor either through a Standard Representation Agreement or otherwise.
(b) If a player, already a party to a Standard Representation Agreement, initiates communication
with a Contract Advisor relating to any of the subject matters listed in Section 3(B)(21)(a) the Contract Advisor may continue communications with the Player regarding any of those matters.
(c) Section 3(B)(21) shall not apply to any player who has less than sixty (60) days remaining
before his NFL Player Contract expires, and he has not yet signed a new Standard
Representation Agreement with a Contract Advisor within the sixty (60) day period.
(d) Section 3(B)(21) shall not prohibit a Contract Advisor from sending a player written
materials which may be reasonably interpreted as advertising directed at players in
general and not targeted at a specific player."
A sample player-agent representation agreement can be found in Appendix D of the NFLPA Contract Advisor regulations.