"To join the effort" means to actively participate and contribute to a collaborative undertaking or task. It implies that someone is becoming part of a group or team, and will work together with others towards a common goal or objective.
Consider offering opportunities for customers to join those efforts through in - store and online matching donations and special events. (petbusiness.com)
We ask all parents and youth to join efforts advocating for these changes, and we urge voters to elect representatives who will take effective action to prevent gun violence in our nation. (ate1.org)