Secure media downloads

I am trying to add a membership plugin to a wordpress site that will allow a post,page and file to be assigned to a group. The file assignment seems to be the most difficult to find.

Is there a way in WordPress to secure a file so that only specific groups of users can download the file. I have looked at several plugins and not found something that can do this easily. I am new to WordPress and would really appreciate any advice.

We are willing to pay for the right plugin, but have to be able to proof it will do what we want first.

I am currently looking at WPMU Membership, have look at s2Membership also and a few others.