Within your selected loop, you can add anyone to the transaction. Adding a person to a loop does not give them access to anything in the loop, it just simply allows you to have them in the loop for when you're ready to share documents to them or introduce them to someone else in the loop.
This option also exists in the View Details section of your loop (See Step 5 of this article to learn more).
Note: You can only use one email address per person within a loop.
Note: when adding someone to your loop team you must understand that they will have full access to everything in the loop, and that once the invitation is accepted you will no longer be able to share anything to this person (because they already have access), but most importantly, this cannot be undone.