why can't other users view content posted by user? Where can I find this permission issue?
Go to admin/people/permissions and ensure that "View published content" under the title "Node" is selected.
Also, if using OG go to admin/config/group/context and ensure that "Node" is enabled, and that if you have "Content" under admin/config/group/permissions that these permissions are also correctly selected.
Also ensure that the node you want all users to see has the two checkboxes Published and Promoted to front page checkbox ticked. You find these under "Publishing options" when you edit the node as the administrator.
The defaults for these are set per content type. If a user that is not the administrator is creating content, these must be set by default for the type. The built-in content type "Article" has these checked by default, so the user should create article of type "Article" (or another type that has these two boxes checked by default) for the article to be visible on the front page for non-admin users.