The method described here: Retrieve a user's Display name from their login (as well as on similar posts) seems to work, but the example uses only 1 ID in the people/group field. When I attempted to use it for a workflow which used a people/group field which allowed multiple IDs, it did not work. In fact, I could not get the LDAP method to work at all. I could only get the web service method to work, but only for a single ID in the people/group field. Note my comments at the bottom of that article. Any assistance is much appreciated.
(BTW, as noted in my comments there, compared to the ease of how this is accomplished in SharePoint Designer, I was surprised it is so difficult in Nintex workflow. So, I've added it as an enhancement request. See Ability to Quickly Select Ouput 'style' of 'display name' for People/Group Field in Send Notificatio... Please vote for it!)
I had something similar here How do I get the manager for more than one person in a single people picker field?
You should be able to use those ideas to help?