A person who represents "all" similarly situated contractors, or none of them specifically, would not need to register since he would not necessarily be acting on "behalf of" any employer or real party in interest. This interpretation, however, is going to depend on the nature of the relationship between "all similarly situated contractors" and the association representative. This interpretation would be different if the trade representative was paid by the association members for the purpose of engaging in executive branch lobbying.
However, if the trade association representative is actually employed by a particular company who would stand to benefit from the changes in the bid specifications (improves its bargaining position for the next contract), then the trade association representative should register and list his or her particular company as the Real Party in Interest—even though he may be also acting in his capacity as a representative of the trade association.
For more information, see the Executive Branch Lobbyist Registration Rule or call (317) 234-4431.