Learn Icelandic: Hann heimsækir þá oft.

The verb heimsækja (to visit) needs its object in the accusative case. Because he is visiting a group of men, we use the masculine plural accusative pronoun þá. If the group were all women, it would be þær, and for a mixed-gender group, þau.

I thought "they" was þeir for a group of men. Why is it different here?

Þeir is the nominative (subject) form, used when "they" are doing the action. Here, they are receiving the action (being visited), so we must use the accusative (object) form, which is þá.

Is the verb heimsækja made of two different words?

Yes! It combines heim (home) and sækja (to seek or fetch). In Icelandic, "seeking home" to someone is how you express the idea of "to visit."

Can I put oft (often) before the verb like we do in English?

Usually, no. Unlike English where we can say "He often visits," Icelandic generally places adverbs of frequency like oft after the verb and its object. So Hann heimsækir þá oft is the standard and most natural order.