Hello Everyone

We are using Nintex 2013 and 'm facing an issue about user rights

I have a stakeholders list on each site attached to each projects, I created an approbation workflow to approve some items in the Risks list, the workflow reads the person in the stakeholders list and for some of them, it sends them an approval request

But some of my users are not in the site group member so when they try to open the approval url, they get the following message: 

Sorry, something went wrong 

Cannot find the task. It may have been deleted, or the workflow cancelled.

I tried to change the items right in the workflow but I can add full rights nothing changed, the same error message

So I guess to be able to approve a change request, my stakeholder has to be part of my team member right? If it is the case, is there a way to achieve that via a Workflow? Because I can't do that manually (every day more than 50 sites are created) and I don't want to deploy a event receiver due to the amounts of internal procedure to do this kind of job

Thank you for your help