A user can be assigned one of the following roles
Reader - Has read-only access to the Web site.
Contributor - Can add content to existing document libraries and lists.
Web Designer - Can create lists and document libraries and customize pages in the Web site.
Administrator - Has full control of the Web site.
Can more than one person use the same login?
If the users sharing that login will have the same permissions and there is no fear of them sharing a password, then yes. Otherwise, this is discouraged.