I found this quote from Bereshit Rabbah 84:4: Ya'akov, his father, made many converts in Egypt. And another in Shemot Rabbah 18:10 G-d made a joyous occasion for Israel, since He redeemed them - God said, "Anyone who loves My son should come and rejoice with My son." The proper ones of Egypt came and made a Pesach sacrifice with the Jews and went out with them, as it is stated (Exodus 12:38), "And also a mixed multitude went out with them."
Often these groups are later depicted as not sincere and troublemakers like the Erev Rav for example.
But are there also commentators or sources who teach anything about those who actually converted completely? Who during the time that the Israelites lived in Egypt got connected/attached somehow and through all the miracles and plagues they witnessed got to the insight that there's only one G-d (they came to know that HaShem is Lord (Shemot 7:5, 14:18, 29:46; see also Shemot 18:8-12 what such a witness could mean). Where there people who truly accepted G-d and joined the Israelites in their servitute to Him?
Did any of the good which befell the Israelites, also befell other righteous persons living in Egypt at the time?