SharePoint permission levels

Question

How do permission levels work? I don't know why I have "access denied" every time I access a subsite even if I have read permission. I am hopeful that somebody will shed light on this.

Here's the scenario:

Intranet (parent site)
|- Intranet Owners (Administrator)
|- Intranet Members (User1, User2)
|- Intranet Visitors (GUser1, GUser2)
|- HR Members (User1) // Contribute permission
|- Corporate Communications Members (User1,User2) // Contribute permission
|- IT Members (GUser1, GUser2) // Contribute permission

Subsites will be the departments:
|- HR (default group - HR Members)
|- Corporate Communications (default group - Corporate Communications Members)
|- IT (default group - IT Members)

Here's the question:
Can GUser1 access the Corporate Communications site? GUser1 has read permission on the site collection level. This is the problem I'm facing right now. I logged in as GUser1 and got an "Access Denied" error. Why?

Answer 1

If you're saying that GUser1 has Read permission at the site collection level, depending on how the subsite was created, they might not have access to the communications site. Check the permissions for corporate to ensure the Intranet Visitors group has read permission. By the way you have spelled it out, it does not appear that Guser1 will have access because he is not listed in the Corp Comm users group and you haven't listed Intranet Visitors as having access to the Corp Comm subsite.

Just because a group as permission at the root of a site, it does not mean that it trckles down to everywhere in the collection. If subsites are created with custom permissions, then yhe permissions inheritance is broken from it's parent, and any groups that exist at the parent need to be explicitly added to the subsite.