Just discovered these little critters on my bonsai. I am assuming they are spiders because the bonsai has webs on it. Are they harmful? How do I get rid of them?

There are lots of solutions. I use neem oil, bit others kill, too.

Make sure you get something that kills spiders. They are not insects so insecticides may not work.

is your tree inside or out? Inside these bugs seem to thrive much more ( and the tree needs to be outside anyway)

This looks like spide mite. These thrive under dry windless conditions. Putting it in a spot where some airflow is present and ensuring higher air humidity can help reduce the impact. Frequent showing of the foliage can reduce the infection. In this stage I would go for a miticide though.