Basically consumer role have readonly right on the objects.You can use that role to give read only access.By default user's have ready-only most of stuff except folder inside userhome. If you are refering to sites, than site consumer have same role.
Create simple user from site dashboard and give Consumers role to the user, Consumer has read only-rights in a site: they cannot create their own content.
For more details about users role and permission I'm attaching a link for you.
