You need to fully secure all the NFS clients. NFS' security model relies on being able to trust the clients to manage UIDs and access control. There's a minimal amount of root-level security in
root_squash, but any non-root user is fair game for an NFS client to abuse if they have a mind to do so.
Once you have got NFS clients you can trust, the problem is the same as any other "multi-user web hosting" scenario -- a set of permissions that provide appropriate access controls to read and write files as required by the operational requirements of the system.