In the various conversations I've had with religious people in my life, I've often heard one commonly cited "pre-mortem benefit" (my terminology, not theirs) of membership in a religious group: the sense of community they feel among members of their congregation. It seems to me that it's more than just being surrounded by people who reaffirm their beliefs; people in religious congregations can serve as a network of support (material and/or emotional) for one another during times of crisis, provide an ample supply of volunteers for service projects, and simply present another venue of opportunity for forming new friendships.
Non-religious organizations focused on common interests and hobbies can do some of these same things, of course, but it doesn't seem like it happens on quite the same level. I found that sense of community to some degree as an undergrad in Phi Sigma Pi (a national co-ed honor fraternity, through which I met several good friends, including my wife), but in all honesty I still felt just slightly out of place at fraternity events, and I haven't felt motivated to stay involved as an alumnus.
In the past I've considered joining a Unitarian Universalist congregation - the idea of interfaith cooperation and a search for truth across different theologies certainly has appealed to me, particularly back when I called myself an agnostic. To be honest, however, it just doesn't seem like something I'd enjoy in the long run; I'm really just not interested in feigning credence toward spiritualism, and if I'm going to commit my free time to an organization, I'd rather not feel like I'm wasting it on something I just don't believe in.
In the past year I've become aware that there are a growing number of secular humanist organizations out there for people who want this sense of community without the mythology. These are groups of atheists and agnostics, centered on the philosophy that human beings should help one another find truth, health, and happiness in our material world without needing to speculate about gods, spirits, or an afterlife. I'll likely be paying a visit to the Humanist Community of Central Ohio in the near future to see if they're right for me. I'm also planning on going to an atheist Meetup next week in order to (for lack of a better phrase) meet up with some other atheists in Columbus. Maybe I'll feel right at home with the group, maybe I'll feel like an outsider who just shares their skepticism toward religion. There's only one way to find out...