Permission to edit a node by multiple users
The best option is to install a node access module that allow the admin to:

Create access control lists (ACLs) to individual nodes; and
Add and remove individual users from an ACL.

The module named Flexi Access lets you do exactly that.

The Flexi Access module will provide a simple and flexible interface to the ACL (Access Control List) module. It will let you set up and mange ACLs naming individual users that are allowed access to a particular node.
The module let you manage access to individual nodes. The administrator may grant individual users the right to: View (read), update (edit/write), and delete the node.

It depends on the ACL module.  (It actually provides an administrative interface to ACL.)
Disclosure: I am one of the maintainers of Flexi Access.