Grant specified permission(s) to a role.
drush role-add-perm
role : The role to modify. Required.
permissions : The list of permission to grant, delimited by commas. Required, unless the --module option is used.
--module : Select the permission to modify from an interactive list of all permissions available in the specified module.
Allow anon users to post comments.
drush role-add-perm 'anonymous user' 'post comments'
Allow anon users to post comments and access content.
drush role-add-perm 'anonymous user' "'post comments','access content'"
Select a permission from "node" permissions to add to logged in users.
drush role-add-perm 'authenticated user' --module=node