It depends on what you wanna do but for our school, you get to choose which core field you want to do your sub-I in (ie family, med, peds, surg) and then from there, you can choose your site and group. I dont think you can do your sub-I in a field like anesthesia, ortho, for example.
If we are talking medicine or peds where attendings rotate on/off service fairly frequently, it would require knowing the attendings' schedules.
If we are talking surgery or a surgical specialties, the attendings are pretty much always on, so if you select their service you will get to work with them.