I need to be able to use a create trigger to create another entity.
Example: when a Meeting entity is created, for each attached user create an Invite entity, which they are notified of.
If a Meeting is linked to a set of Users (e.g. via an Attendees field) then that implies that each User is linked to the Meeting (via a Meetings attended field or whatever).
Instead of using the creation of the Meeting as the trigger, why not use an automation in the User database, triggered by the linking of a Meeting to a User.
In this way, the automation will be triggered for every User.
You really have to use @Chr1sG 's suggestion, because when a Meeting entity is created there will not yet be any Users attached to it, so a trigger at creation time will not have anything to process.