Do my clients need to subscribe to GOrendezvous to be able to book an appointment?

They will only be required to input their contact information if they find an appointment which they wish to book. If they don't find an appointment which fits their needs, they will not be required to give any of their personal information. 

See the demo here: https://www.gorendezvous.com/en#clientbookingdemo

The clients who wish to combine their personal appointments on Google Calendar with the availabilities of their professionals will have to have an active GOrendezvous account to be able to integrate them with their Google Calendar schedule.