You actually answered the first part of your question in your comment on this post, while responding to my incorrect suggestion that the user should be a member of the sitecore domain to access Sitecore. I have updated my answer with the correct information and my recommendations for your needs, below.
Adding Access to the Sitecore Client
The Sitecore Client Users role is all that is required in order to have access to Sitecore. Being a member of the sitecore domain is actually not required.
According to Sitecore's documentation, the Sitecore Client Users role adds the following:
Gives the user minimal access to Sitecore. With this role, the user can log in to the Sitecore Desktop, but will not have access to any applications.
This means that so long as you add the Sitecore Client Users role to a user, said user should be able to log into Sitecore but will not have access to any of the applications.
Adding in Access to the Content Editor
The next step is to give your user access to the Content Editor. I recommend adding the Sitecore Limited Content Editor role, as it grants access to the Content Editor with the minimum rights granted. Note that if you add this role to your users, you don't actually need to add the Sitecore Client Users role, since it is included in the Sitecore Limited Content Editor role, as well.
Per Sitecore's documentation, the Sitecore Limited Content Editor role does the following:
When a content author is assigned this role, they only have access to the Home, Review, and Publish tabs on the Content Editor ribbon and have no access to copy, move, or sort from the item's right-click menu.
Adding Ability to Read Specific Items
Lastly, you should create an extra role that has read access to the items that your limited users should be able to read. Give that role to your users, as well, as the Sitecore Limited Content Editor role and your should be good to go.
Remember that for your content authors to be able to read an item, they should be able to read that item and its ancestors. The only exception to this is if you have the ContentEditor.CheckSecurityOnTreeNodes setting set to false. If so, then your authors will be able to see but not click on items in the tree that they do not have access to. See this post for more details on the ContentEditor.CheckSecurityOnTreeNodes setting.
