Not working for our singing circle

Threshold Choir is an international organization that trains singers to visit dying patients and deliver peace and comfort (and sometimes pain relief) in the form of simple, closely-harmonized lullabies. I volunteered to test Flock to see if there was a chance we could use it to learn and rehearse harmonies online.
Several things have affected my decision to suspend testing for now. Our membership skews older as most of us are retired, and therefore we tend to not have as much tech savvy. This is also true for our chapter directors, most of whom are amateurs or at least, unpaid pros. When our members do have a computer it is more likely to be a Windows PC than a Mac. Our groups are very small…we send 3-4 singers to each bedside, and each individual needs to hold their part against the others; singers often change which part they sing from song to song (so the traditional “section” divisions are not only irrelevant from unwieldy). I was able to get Flock loaded on my PC and do a successful audio test (using the audio extension) but couldn’t make a session with one other singer on a Mac work. I uninstalled and reinstalled but couldn’t get as far as I did with the first install. The other hindrance is the lack of video: we synch our words by watching each other’s mouths.