You bet, you can invite anyone to read or edit your screenplay

The invitation process at Script 360 is based (for now) on email addresses (for now), so it doesn't particularly matter if the person you are inviting is a Script 360 member or not:

Non members go through an extra security step

To ensure that invited non members are who they really are, anyone using the invite link we send when they accept your invitation may need to enter a one time password sent to the same email address. This should prevent anyone from passing around the invitation link to others who does not have access to the invitee's email account.