OK, let me understand your scenario:
You have a (seemingly) public facing fileshare that is encrypted.
You add more people to it on a semi-regular basis
In order to add the keys of new users to be able to access it, you're having to re-encrypt.
Well, from my knowledge of netshare, if you add a user to an encrypted folder, it has to be re-encrypted, I cannot see a way around that. So the next question would be do all these users need access to all the information in this share? Can is be segregated to slow down these encryption times?
The management server does not offer a lot in terms of the NetShare product, its much more orientated around email and full disk encryption, the fileshare folder is still very much endpoint heavy.
You could install the product on the fileserver itself, which will offload processing power and will eliminate any network traffic.