For Apple calendar events Apple does not give us access to modify attendees - either adding, editing, or deleting them - from an iOS event. So for Apple events we can't do that until Apple allows us to.

In the meantime you can set the settings in Informant to create events using the iOS editor instead of Informant's own editor.  

When you do that you are given Apple's iOS Event Editor instead of ours, and because it's Apple's editor it is allowed to modify/edit/add attendees.

To change this setting so that Apple's event editor automatically opens go to the Informant Settings > Accounts > Options, and turn off both sliders for the Informant Editor.  This will change Informant to use the iOS editor and the invitee field will be visible.