Restricted Access/Private Portal

I’m in the process of writing a proposal for a site that wants to include the following (excerpted from the RFQ):

Provide a portal for Board members, partners and staff (both in-house and contracted) that will have information tailored specifically for their knowledge. The portal for the Board members should be password protected.

I’ve seen a LOT of posts about restricting access for members, but since the client isn’t interested in creating memberships, I’m wondering if there is a quality plugin (paid or free, isn’t an issue) that could do something like this. Although the RFQ doesn’t mention it, I imagine they would want users to be able to create their own usernames and passwords.

I know some PHP, but not nearly enough to approach this problem from that direction. Any suggestions would be greatly appreciated!