You played an "integral" role in the formation of the team
integral adjective: 1. necessary to make a whole complete; essential or fundamental. "games are an integral part of the school's curriculum"
synonyms: essential, fundamental; Google integral
If "integral" lacks the prominence you seek to convey then, perhaps, "essential" is more serviceable:
essential adjective: 1. absolutely necessary; extremely important. "it is essential to keep up-to-date records"
synonyms: crucial, necessary, key, vital, indispensable, important, critical; Google essential