Please Please help a Newbi

Hello,
Apoligies for my ignorance here I do not usually look after the webmaster side of things but are currently learning it I am looking at uploading 2 sport membership sites and running them through authorize.net to my merchant account. What we would like to know is can we do as follows.


Clients go to our site => Like what they see and decide to join => Click join and are taken to the url where aMember is hosted (different site to where they came from) => Enter CC details etc and click submit => authorisation is given and they are then given the pass... to the site they came from.

Can we host aMember on site 1 to give mebership to our site #2 or #3 wich ever site they wish to join and does the script have a code that it will know what pass to send to what site and is it hard to install or can a newbi like me have a go??

Again apoligies if this seems stupid, I am only new and are assesing what is the best option for our sites.

Thank you for your time and Kind regards

Please ask for further clarification if this doesnt make sense