This mass of Jewry that accompanied the Jews who left Egypt the Torah calls Erev Rav. They were a mixture of foreign-minded Jews who, at the time of exodus, stood distant from devoted Jews. They were truly from the lineage of Avrohom, Yitzchok and Yakov, but grossly assimilated. They were a riffraff of arbitrary and whimsical thinkers.
Even today we have such a bunch, and when Moshiach will come to redeem the Jewish majority he will surely apply the same name, Erev Rav, to those Jews torn away by Communistic doctrine, anarchist rationale, socialistic beliefs, or break-away mindsets, whom today Hitler is chasing back into their ranks! Among these we can include the Reform movement or other torn-away heretics. If after the birth pangs of Moshiach such Jews remain, they will probably want to follow all the Jews. They will certainly be called Erev Rav, and Moshiach will certainly have to let them tag along, just as Moshe had no heart to stop them from tagging along.
This is how we must understand it, and are permitted to understand it, because nowhere does Torah say among those who left Egypt more than half were Gentiles. Had, in fact, a large mass of Gentiles actually accompanied the Jews, our sages would probably have created a separate tractate and called "Masechet Erev Rav".
The fact is, our sages do not call this group attached to the Jews as "Gentiles"; They use the specific term Erev Rav!
Everything Torah tells us about the generation in the desert simply does not dovetail with an exodus that comprised a majority of Gentiles.
Regarding several rabble-rousers or malcontents, Torah conveys to us their stories, telling us they were Jewish. The spies were Jewish. Korach and his company were Jewish. Datan and Aviram - Jewish. According to those who would cast several million Gentile bastards among 600,000 Jews, would mean that only Jews were the bad ones and trouble-makers, whereas the Gentiles happened to be Moshe's most loyal adherents; After all, Torah never reports (except the single incident of the Ish Mitzri) how any convert made so much as a peep against Moshe! How odd that Torah deems so worthy to tell about all those "sinful" Jews, as their misdeeds are repeatedly told and recounted, yet never does Torah see fit to breathe one word about the millions of "righteous" Gentile converts!
Certainly Moshe felt grief from his Erev Rav. A third of Jews, at most, were faithful Jews, believers in the ways of their forefathers; Two-thirds, on the other hand, were Egyptian assimilationists and foreign ism worshippers. The latter called themselves "Moshe's Erev Rav" because faithful Jews would gladly have left them behind. Perhaps it's also true God Himself did not want to expressly say these people should be taken along and left it up to Moshe, but definitely these were a Jewish flock.
The Torah speaks nothing of Gentiles among Jews who left Egypt. It makes sense and we can understand that Moshe had no good reason to take along more than 3 million Gentiles together with the 600,000 Jews. But he certainly did have a good reason to take along all Jews without exception, as well as a small number of Kosher Egyptians who submitted to servitude to the Jews. The Holy Zohar states this expressly: It tells us the converts stayed next to the sheep outside the encampments, and these were sustained by their Jewish masters.
We can therefore conclude the Erev Rav were Jewish and among these a small number of converts came along who probably, still in Egypt, befriended some assimilated Jews.
Those who account for converts among those who left Egypt are correct - but they surely do not speak about the entire Erev Rav! R' Yishmael, R' Akiva and R' Yonanssan speak of the Erev Rav and their large numbers without mentioning one word about converts! They meant to say faithful Jews, faithful as their forefathers, numbered no more than 600,000, whereas the majority who left Egypt had assimilated and were distant from the faith. Other commentators who speak of converts furnish no numbers; They speak generally, as did the Rashbi, saying converts lived outside the camps, and for whom Manna did not fall, and that these were sustained by leftovers of the Manna that fell for Jews. From this alone it's understandable their numbers were small - because nearly 4 million converts could not possibly be sustained from leftovers of 600,000 Jews!