If you want to alter a user's permissions to start, view and participate with workflows her is how:

1) Go to K2 Management
 

2) In the left pane drop down WorkFlow Server>>WorkFlows and find your WorkFlow in question

3) Select the Rights tab and you should see something like this:

4) Here you can add basically whomever you want; SP Users/Groups, ADD Users/Groups, SQL, CRM whathaveyou. Just search up whoever you need and click next.

5) Assign appropriate rights:
 

6) Finish and save