Here is an speech on group meditations by Supreme Master Ching Hai, (taken from here). Her followers are responsible for some of the vegetarian/vegan restaurants here in Portland, like Vegetarian House and Sweet Lemon.
Why Are There Incredible Benefits From Group Meditations?
From Master's speech during a group meditation in Japan on July 12, 1992
(Originally in Chinese)
The merit derived from group meditations is many times greater than that of one person meditating at home for days, months or even years, depending on the number of participants in the group meditation. Therefore, the more the participants, and the longer we meditate in group, the more difference it makes to our spirit. Many people I can tell just by their looks whether they go to the group meditations or not. Fellow practitioners are aware of this too. There are obvious disparities. If you attend the group meditations often, don't attend for a period of time, and then attend the group meditations again afterwards, you will feel different, feel that you are not so pure.
We gain more blessings for our country when more people attend the group meditations. Don't need to wonder: How can our nation derive blessings just because a few of us are meditating together? It is true! Just like several people are living in a big house. It is enough if one of them is rich and willing to contribute. He will repair any of the leaking areas in the house and thus benefit the whole family. Even if dozens of people stay in that house, no problem. This would benefit himself, and all the residents would feel secure. Such is the case with one family. In the same way, definitely other people would benefit from the spiritual practice of just the few of us.
Just one light by the roadside, wow! It is useful to so many people, thousands and thousands of passers-by. The lamp loses nothing, but becomes even more functional when more people are using it. Therefore, though many people in our world might not be practicing spiritually, it is sufficient if we do. Otherwise, it would be even worse if no one was practicing at all. Just like when the entire village has no light, there would be total darkness. If we could have one or two lights, it's better than none! The highway is very dark, yet we could see a long distance if there were several lights. So, practice more diligently, benefitting yourselves and everyone. This is what Shih Fu loves most.