You must contact NCH using the same e-mail address used when purchasing the product in order to receive the support service you paid for, otherwise you'll be put in queue for the lowest level support.
I don't know why Quorum won't answer the invite request. I know that some NCH products have had difficulty in the past interfacing with some VoIP providers that don't use the regular method of SIP communication (e.g. add unnecessary tags at the end of a line), after contacting NCH about the problem they discovered why it wasn't working and were able to promptly provide me with a version of the program that could work with that VoIP service.
In terms of concurrent calls, it is completely dependent on the internet connection you have and the specifications of the computer running the Quorum software. If you are planning to have many people call in at a time at a high quality, you will probably want to have a dedicated ADSL2+ or greater internet connection (which can provide speeds of up to 20mbps or so).
If necessary, you can get internet connections with bandwidth/speed levels of up to 45mbit in your (Pleasanton, CA) area. However those speeds come with a hefty price.