On a Multisite install how can I syncronise members between sites or restict access to an entire sit

I have a multisite installation for one of my customers and they have their main site 'Site A' with membership plugin installed and running well.

They wanted a directory element so we have purchased a theme with all directory elements included and have set it up as a new site within the multisite install. 'Site B'

We want to restrict access to the directory 'Site B' to members with a certain subscription from 'Site A'.

I have tried setting up a URL group under access levels for all addresses from http://www.siteA/siteB but this doesn't work.
If I activate the membership plugin on 'Site B' it doesn't link to all the members from the main installation (siteA) and if new members join they aren't added to both sites memberships so I can't controll access by using the membership plugin on the directory.

Can you offer any advice on how to restirct access to the directory.

Sorry if this isn't clear I'm not sure how else to explain it.