the pricing policy you are referring to is HipChat Plus Cloud. Once Atlassian has set up your HipChat Cloud instance, you will be able to manage your users in HipChat. Every named user counts as one license and will be billed $2 per month. You can add and delete users anytime you want.
There is also HipChat Basic which you can use for free and HipChat Data Center which you have to install on your own Server. You can find more information in the HipChat Licensing and Purchasing documentation of Atlassian.
Please let us know if we can assist you any further.