From your Pre-Plan page, this page can be accessed by clicking the Who is helping you with Pre-planning? button.
For each person who is helping you with your pre-planning, fill out their Name, Email, Phone, Relationship to the person helping with planning, and Organisation. You can speed this up by clicking the icon next to a Connection's or Invitation's name to copy their details into the fields. Then hit the Save button.
If you have an email for someone, you can invite them by clicking the Send an invitation to this person checkbox. You can also add a message in the Message field. If you do, the invite will have minimum permissions (that is, Custom, with each section set to No Access) and you'll have to edit the invite or the connection to give the planner the access you like.