When I have no personal relationship to the deceased person, I go to funerals for the sake of my relationship with the bereaved. I’ve attended many more services to support the family than to express my own feeling of loss. I also send a card with a personal note. If I can say something about the deceased loved one, I do; otherwise I concentrate on words of sympathy.
In your case, I would certainly go. Your group is far too small for your absence not to be noted, and it could well be misunderstood as indifference to your coworker’s loss.
@glacial,
My feeling is that there are very few people that a grieving person wants to interact with at a funeral.
That might be something that people see very differently. When my father died as a member of an extended academic community, the large church sanctuary was full. I didn’t have to deal with all those people, but I felt their presence, and it meant something to me. When my mother died, the service was in a small, out-of-the-way town that few people would travel to reach. The attendance was very small, hardly anyone besides family. To me this was much sadder.