I've taught a few free community classes in different settings and here tried to generalize about what seemed to work well. Obviously many factors vary from site to site, and some may be impossible to control. Please feel free to share your own experience setting up your own yoga service class and thoughts on this topic!
1. Independence of set up: The longest-running yoga service class happened very independently in a Chicago west side library. There was a security guard but the indoor space we used was unlocked, it was the same space each time, and under normal circumstances, I didn't need to check in with anyone to start class. It can be useful to have a single staff contact, but if the person isn't there regularly, and you need to interact with someone, it can be time consuming to communicate with a different person each time if they are unfamiliar with the arrangements.
2. Mats & blocks: I stored yoga mats and blocks in my car, and carried them in and out each class. This often required multiple trips (and took up serious space in my car the rest of the week!), but access to yoga blocks is tremendously helpful. In another location where I would leave the mats on the premises, it was time consuming both for me and staff to find the person with the key to unlock them, carry them all the way back to the space, and the mats were sometimes used outside of the class by people wearing shoes dirty enough to leave caked dirt on them. This meant extra wear and tear on the mats, and over time would have required more thorough cleaning than I could have realistically provided to offer students decently clean yoga mats.
3. Consistency: For a long time there were few cancellations by the library (and when there were more it led up to the end of the class). If and when I did need to cancel the class myself, I had collected emails and reach out by email. This may have affected attendance more if it had been more frequent and people could not count on regular classes. Pairing up with another teacher can be helpful in many circumstances, particularly if both teachers are able to attend some classes, learn the other's teaching style (and likely share it), but I do think part of the reason people return to my classes is that they have an idea of what they will be like. A class that is led by a different teacher each week would likely not have that similarity week to week.
4. Attendance: An RSVP system was not helpful. I would never want this to be required, at least, not in any yoga service class I've come across, because plenty of people who could benefit from free yoga may not have regular email or internet access. Plenty of people will come but not RSVP, and of course, even if people do RSVP, plans may change. Ultimately having a big enough group of regular students will lead to classes that have at least a handful of students each time, even if the students are different week to week. The library class ultimately went on for more than two years, and there are students who came to several classes, took a long break, and came back - people's schedules do change and it's realistic to consider that as a reason for lack of attendance rather than anything within the instructor's control.
5. Student diversity: People of a similar age range and ability level is helpful too. Obviously this is not something a teacher or organization can control much of the time, aside from noting that a class is only for adults or only for children. I always try to teach to the midline of the group, and offer enough options that at least one seems suitable for each person. I do try to describe the class in any promotional materials so people can come with an idea of what to expect (students should be comfortable sitting on the group and rising up to standing; while this isn't a chair yoga class, chairs are available if you'd rather sit on a chair than the ground).
6. Support staff: Happening upon a senior staff member who is supportive of a yoga class is important, if at the very least for securing space. The library probably differed from many non-profits which may be more common yoga program hosts. In those cases, it seems closures for events or holidays might have been more of an issue, in which case having a staff member who will coordinate with the instructor (and potentially with students if they are organizational clients) would be even more essential.