I don't normally ride on MUPs but do use a 1 mile section to transition under a highway. Riding today, when I came across a family with 2 little kids I slowed down to walking speed and made my way around the kids. Ringing a bell in that situation would just be annoying to the family. A little farther along there were two women walking away from me on the right side. I took the far left side of the path with at least 6 ft between us. Again a bell would have been unnecessary and annoying.
I find it's better to just ride at an appropriate speed and not expect everyone to be jumping out of my way when ringing a bell.