Many group project administrators require Join Requests because they like to screen prospective project members. During the screening process, they are able to collect known genealogical information such as earliest known ancestor and geographic origins.
However, when you require a Join Request, you risk perspective project members being discouraged from joining your project. They may even feel discouraged from ordering a DNA test at all. If you require Join Requests, then you also need to be able to respond to them within 48 hours. This will ensure positive service to your prospective project members.