How do I get the Ustream-Bot to join my chat?

The Ustream-Bot automatically joins channels once they have a specified number of users and parts when there is less than that number of users for several minutes. The number changes based on the current server load. So, you don't need to do anything to make it join.

By the way, most of the Ustream-Bot commands can be used even if the Ustream-Bot isn't present in your channel so it's not necessary to have it in the channel.