The owner of a Shared Jar - that's the person that created the Jar - can set the permissions for anyone added to that Shared Jar.
There are various permissions to choose from; whether other people can spend, and if so, how much they can spend. This is called a spending limit.
If you are invited to a Shared Jar, be sure you are happy with the permissions you are given before adding money. As you may not be able to spend from that Jar.