Add an invite button and allow the user to login to the campaign. Either by: a) reworking buyer access to campaign by a direct pivot table to campaign, or b) automatically making them and the others into an individual(s) entity (if they aren't already one) and assigning the new or existing entity to the campaign which can be updated to other entity types later, or c) Asking them for the entity structure now. The details will need to pre-load into the authority 2 act to ensure there is no double entry etc.