What's the best way to propagate baker's yeast?

Absolutely there is a way to propagate yeast, it's as simple as making a starter with it. Most of the time these days people create starters for sourdough using natural yeasts, but you can use them to feed any kind of yeast. All you would do is put flour and water in a container with some yeast, let it get to work and once you start to get bubbles put it in the fridge to slow it down. You would then use half of it for every batch of bread you make, replacing the flour and water to keep it at the same quantity.

If you don't use any of it for a few days you'd want to throw half away and refill in order to give the yeast a fresh supply of food. You can share this with as many people as you like, just split it into two, give each a fresh supply of flour and water, and repeat as many times as necessary. Using this technique a small amount of yeast could theoretically supply the entire planet.

NOTE: During the Coronavirus outbreak it's worth noting that corona can survive on surfaces for quite some time, so you should take great care to ensure that your yeast propagation does not lead to virus propagation!

Because I have been home brewing for about 30 years, I do it a different way. I sterilize sugar and water by boiling, cool to room temperature, pour into a sterilized 1 gallon jug, put a little bread yeast in it, and put a rubber stopper w an airlock on top. When it’s done, I pour off most of the nasty beer (sugar only with bread yeast makes a nasty brew though ymmv), I swirl the remainder to mix it and then pour it into very small containers, at the bottom of which will settle good yeast. The alcohol will keep it good for months in the fridge. No bacteria, no wild yeast, no off flavors. No need to feed the starter. When ready to use a little yeast, just pour off the alcohol and put the yeast where It needs to go.