1 The Hebrew text has the article prefixed to the noun. Here the article indicates the generic use of the word אָדָם (’adam): “humankind.”
2 This disjunctive clause (conjunction + subject + verb) is circumstantial to the initial temporal clause. It could be rendered, “with daughters being born to them.” For another example of such a disjunctive clause following the construction וַיְהִיכִּי (vayéhiki, “and it came to pass when”), see 2 Sam 7:1.
3 The pronominal suffix is third masculine plural, indicating that the antecedent “humankind” is collective.

In other versions

When man began to multiply on the face of the land and daughters were born to them,

When human beings began to increase in number on the earth and daughters were born to them,

When mankind began to multiply on the earth and daughters were born to them,

Then the people began to multiply on the earth, and daughters were born to them.

And it happened that, when humankind began to multiply on the face of the ground, daughters were born to them.

And it came to pass, when men began to multiply on the face of the earth, and daughters were born unto them,

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