How to implement content access for groups of users?
The Organic Group module will come to your rescue. Read the documentation about how to use the module.
More about the module:

Enable users to create and manage their own 'groups'. Each group can have subscribers, and maintains a group home page where subscribers communicate amongst themselves.
They do so by posting the usual node types: blog, story, page, etc. A block is shown on the group home page which facilitates these posts. The block also provides summary information about the group.
Groups may be selective or not. Selective groups require approval in order to become a member, or even invitation -only groups. There are lots of preferences to configure groups as you need.