Invitation emails will only be sent out once per user. Regardless of how many times the enrollment is removed and added back in unless the group or user is enrolled again via the "Re-enroll" feature
If the group was re-enrolled with an invite they will get the invite again.
If the group or user was re-enrolled without an invite being sent then if the group or user enrollment is removed and then added back the users will get an invite since the previous enrollment did not contain the invite.