Who can see and approve requests on Turbine?

Visibility

Individual employees can see their own requests.
Managers can see any requests from people teams they manage.
Viewers can see but not approve requests for people in teams where they have been added to the list of 'viewers'.
Admins can see every request on your account.
Specialists in the finance or purchasing teams can see approved expenses and purchase requests. (Add members of these teams in the Roles and admins page on the settings menu.)

Admins can approve any request.
Team managers can approve any request from a member of a team they manage, including other managers on that team, providing the approval permissions in that team allow them to do so.
By default, individuals can't approve their own requests.
You can set up two-stage / two-person approvals. See multi-stage approvals for more information.

When it comes to marking an approved PO as received and/or completed:

Admins can receipt and complete any PO
Team managers can do so for requests from members of teams they manage, providing the team approval permissions allow them to also approve the request.
Specialists in the purchasing team can see approve purchase orders and mark them as received or completed. (Add members of these teams in the Roles and admins page on the settings menu.)
Individuals can also mark approved POs as received and completed.

If you can see a request, you will get an email notifying you about changes to its status, for example when it is approved or edited.