A user would sit in the invite more button for 1 of these reasons:
The original instructor (or manager who created on behalf of the instructor) who sent out the request did not invite certain instructors when they first sent out the sub request. If you allow personal invites to be sent, instructors can send invites to a selected group of people out of the total eligible instructors and deselect those they do not want to send the sub request to at that time. The rest of the instructors who were not invited will sit in the "Invite More" button until someone (the original instructor who sent out the sub request or management) invites them.
If you have availability on, the instructor may have been unavailable when the sub request went out. Then they may have recently updated their availability where it matched the days/times of this request. Those instructors will sit in the "invite more" button until someone (the original instructor who sent out the sub request or management) invites them.
If you have secondary skills enabled, the instructors who did not match the primary skill of the class but did match the secondary skill will sit in that button until someone (the original instructor who sent out the sub request or management) invites them OR if the sub request is still open or created within 24 hours of the class time, the instructors sitting in the "Invite More" button will be automatically invited to the request at that time.
The instructor was new and was added to NetGym after the sub request was sent out but the checkbox "invite instructor to open/pending sub requests" was not checked. This is usually checked automatically when you add a new person or update their skills/locations but sometimes managers don't know what it is so they may accidentally uncheck it. If they weren't added to previously sent requests they are eligible to teach, they will see in the Invite More button.