Why were they called erev, the same word that we use for “evening”?
Both words derive from a root that means to mix. In the case of the evening, it is the period between day and night and is more or less a mixture of both; the day is not completely over but nor has the real night begun.
The two words also remind us of the Hebrew (and English) word for an Arab, though there the root may be different and be linked with a word for a desert-plateau.
The most serious problem is why there should have been non-Israelites who wanted to join on to the Israelite tribes.
The answer is that Pharaoh’s Egypt persecuted many segments of the population, not just the Hebrews. Every group that is suffering from persecution wants to leave. In this case it must have been the Israelites who had the leadership to co-ordinate them and take them out.
No wonder that others saw their own chance in becoming fellow-travellers.