Encrypt files on FTP server

I have a practical situation here, my company is a FTP server provider for some clients. The clients are sharing their files on our FTP server and have had no problem with access management and privacy policies so far.

Recently, our clients want to encrypt their files for one reason: " They don't want FTP server admin has access to their files".

Also, our company wants to enhance our FTP authentication method with digital signature. I know we can implement PGP encryption for sending file to FTP server, but the problem is when encrypted files receive, they will be decrypted by server with its private key and then the Admin have full access to those files.

By deployment of Public Key Infrastructure, each client needs to have a smart card in order to log-on to FTP server with digital signature. But it is not possible for our clients to PGPed their files based on receiver's public key. Because there might be a situation that there is several file recipients.

So, My question is: "Is there a solution to encrypt files on FTP server which can be deployed in this situation?" Any help would be greatly appreciated.