A well-known phenomenon here at CMU SCS is the NewellSimon-Wean barrier. There are several sub-departments of SCS-- including Computer Science, Machine Learning, Language Technologies, Robotics, Human-Computer Interaction, Software Research, and probably others I've forgotten. CSD, MLD, and ISR are in Wean; LTI, HCII, and robotics are in NSH. (Then there are students with offices in Doherty or the CIC, etc) There is a covered bridge about 20m long connecting the two buildings.
And yet somehow I know disproportionately more students in CSD, MLD, and ISR than in the others, even though LTI and Robotics have more overlap with my department in terms of research interests. I think this has to do with socializing factors. The NSH departments have their own lounges, where all the departments in Wean share a lounge (ISR and MLD are both fairly small). Each department has their own social organization to some extent, but the all-SCS social organization, Dec/5, is mostly CSD and ISR people (with growing MLD representation). Even though all of our events happen in Newell-Simon, and I believe our happy hours are well-attended by both buildings.
Of course, anecdotal evidence reveals that Dec/5 participation has a lot to do with personal connections. It is a time commitment, after all, and it's very easy to flake out on volunteer organizations because any given graduate student is "too busy". It's not so easy to do that if your best buddy is in the organization too and will have to pick up the slack. While we get a lot of great volunteers toward the beginning of the semester, once November/April hits it becomes very difficult to put on a TG (happy hour) and for the most part only people in the central "clique" sign up to help out-- and usually out of peer pressure. I also recognize that if I'm not friends with people I'm volunteering with, even if I like them as people, I'm going to get kind of bored.
This makes me think that the key to retaining Dec/5 volunteers is to integrate them in quickly though separate social activities. If they can become friends with existing committed folks, they're more likely to become committed themselves.