First off and most importantly, the person you wish to assign the RFP or Profile to must already be a user in the system. To read more about how to add a user to the account please read the Help article here.
Once your team member is added to the system, they'll need to click on the invitation you sent them and set up their account. Then you will see them in your list of users.
To change the POC to another person:
1. Click on and open the RFP or Proposal desired
2. Select the Details tab in the upper right hand corner
3. Select another user from the drop-down menu in the Details page
4. Save changes and update