You are able to install vistime on as many machines as you see fit. However, your license will only allow you to log into vistime on one machine at one time (1 concurrent login). You are then entitled to create as many conferences within your profile as you like, once you log out and log in at another machine those conferences will re-appear. That said, you may only activate 1 conference at any one time (unless your license permits you to activate more).
You are then able to invite guests (or seats) within your conference. Generally you will be able to invite up-to 5 participants to your conference (unless your license permits you to invite more participants).
For a full list of the licensing allowances please click here.