Many physicians and many patients have it backwards.

Diseases (all of them!) were initially described by physicians who had no tests. They learned from their patients about their symptoms, they watched them to see the signs. With time better tools were developed which enabled physicians to examine the patient.

Those tools became more and more complex and enabled not only a better description of the illness, but also a better understanding of its mechanism and hence better treatment. They also enabled much less experienced physicians make a diagnosis quite easily. So, the art of medicine was gradually abandoned. Why bother to examine the abdomen or listen to the hearth sounds if you can easily order an ultrasound or cardiac echo? It also made everything much more efficient. You no longer have to sit hours and talk with the patient, or sit by his bed-side watching the signs, you can just order a bunch of tests and send him on his way.

But, we started to worship those tests. So, we no longer listen to the patients or to ourselves, we don't watch them to see the signs, we do a very brief examination and then we order more and more and more tests, until we are lost in the woods and no longer see the trees.

Many patients and their families also think that there is nothing wrong with them if all the tests are normal. They insist on more and more tests, which many times are as normal as those that were done previously, or even worse show some abnormality which doesn't point to any direction (because about 5% of completely healthy people will have test results which are not within the normal range, as this is the way in which the normal range is defined).

Also, those existing tests can recognize only a certain percentage of patients with a known disease (there is no test which is positive in 100% of the patients with a given disease) and obviously can't recognize a new and not yet recognized disease.

MuSK MG is one of many examples for this:
Until MuSK antibodies were discovered in patients with MG, such patients had completely normal tests.
You can argue that their illness could have been diagnosed with other tests used to diagnose MG, but the SFEMG is not diagnostic in a significant percentage (it is hard to know how many, because there may be quite a few patients with MuSK MG who were not diagnosed as MG, but as "something else", more times than not from the field of psychiatry), The tensilon test is not considered an "objective" test by many neuorlogists and it too may be undiagnostic in MuSK.

So, if you are a physician who worships tests, you end up having patients with symptoms and signs of a disease, but they are not ill.
What do you do with those patients?
You either treat them as "suspected disease" (which is bad, because your lack of conviction regarding their illness leads to a hectic management approach which may be worse than no treatment at all) or show them the door (which is as bad).

So, the answer to your question is that patients with MG and normal tests are diagnosed if they are either fortunate enough to reach a physician who doesn't worship tests or fortunate enough to have new tests developed that will be diagnostic for their illness, or both.

How many patients with MG are never given the correct diagnosis and never treated for this illness? it is very hard to know.
I am not even sure where we should look for them-in the CFS community? In psychiatric hospitals? Or do they just somehow carry on with their life and see themselves as "lazy"?