I have a multisite with 4 sites. The first site has links

I have a multisite with 4 sites. The first site has links to the other 3. I want users to have to register for each site individually and use a unique invitation code per site. This is for kids who will eventually move from elementary to middle school to high school. I have BuddyPress and an invitation code plug in. However, if the user tries to go to another site (I put them there manually), I get "Site Access Denied" They cannot even register.