Users with Admin rights of the project can add/modify approvers in the eSignature Menu > Approval Access.
User having Admin rights of project can add/remove approvers for test cases and test suites.
Notes:
On the project details page, there are two expandable panels on the eSignature tab:
Any user(s) having Test Case "Edit" permissions can be added as a Test Case Approver.
Any user(s) having Test Execution permissions can be added as a Test Suite Approver.