Leads Groups provide structure and consistency along with a more formal environment. They typically invite guest speakers to ensure the group is frequently introduced to professionals outside the group.
The meeting consistency of the groups allows you to build trust with your peers, and eventually they will begin sending you leads from your trusted relationship.
Associations are a strategic way to target defined industries to grow your network. Joining associations that have members from industries that can benefit your company specifically, helps to maximize your networking by being target specific. Of course, attend meetings regularly, and there are many ways to get involved like serving on a board, or volunteering to help with events, or working on a committee.