How to keep data from being shared

It is possible, although not necessarily recommended, to restrict access to data to a single member.

If you choose to do this, consider the following operational issues:

You cannot do workload balancing for that data, because the other members of the group are not aware of that data. Thus, the member that has access to the data can become overloaded if access to that data increases over time.
Availability is compromised, because if the member that owns the data goes down, no other member can access that data.
You might have to set up special affinities to allow the application access to that data. Work cannot be automatically routed around the group to find the data.