Pete Seeger was the absolute master at getting an audience of any size singing together. He perfected this ability over 75 years in groups in size from a few dozen on a picket line or in a union hall to Carnegie Hall or a stadium in Spain or Chile.
His work was amazing. He was able to get people singing:
who thought they couldn't or were adamant that they wouldn't
in languages they had never heard before
in four part harmony
with no cues to lyrics other than hearing his voice
even in later life after he essentially lost most of his voice
The video featured here is of Pete at age 94 (at a Farm Aid concert) leading an audience with little or no experience or interest in folk music.