user synchronization - I have mytherapynet.com where people sign up(single wordpress site running di

user synchronization - I have mytherapynet.com where people sign up(single wordpress site running directorypress - which has the therapists' listings) , and then visitors go over to securetherapy.com to visit the individual therapist's Online Therapy Office where they can schedule, pay for and attend sessions. I would like to have anyone who signs up at mytherapynet.com be added to all of the securetherapy.com multi-sites. is there a way to do that? I use multi-site on securetherapy.com and I create all the sites from a template blog - wpmu.