Restricting "view content" to members of a group
Some basics about he Group module to be aware of:


Make sure you enable gnode, a submodule of the Group module.
For each group type you need to define the appropriate permissions (view, edit, delete, etc) for the various Content Types.
Every group can have users, roles and permissions attached to it (refer to "What are the various roles supported by the Group module?" for more details about that).


It seems that you would want to grant access to what, in Group, is called "Members" (similar to what in your question is called 'Group members'). But you woudn't allow access to "Anonymous" (site visitors that are not logged it). However, you also need to think about which access you want to give to "Outsiders" (=  logged in users who are not a member of a group).

With the above in place, it's just a matter of assigning a node to a Group: based on how you configured the roles and permissions, the Group module will allowed/deny access for any user.

More info

Refer to "How to create collections of content that can be accessed by users with various access levels?" for more  info about all this.