> og vildi vita hvað manna væri á skipi þeirra er höfuðburður væri að.

I think he's asking "who are these prestigious people on board",
"who's on board with so much prestige" -- literally "at whom there is
prestige" -- (contrasting them with the run-of-the-mill sort of
travellers who arrive). I'm assuming the relative pronoun 'er' refers
to 'hvað manna' rather than 'skipi þeirra', although either is
possible grammatically, as far as I know. There are three other
examples of 'höfuðburður' in the database, one referring to an
inherited quality, one to prestige attained (or not attained) through
action, one to prestige derived from location.

1. og þótti mikill höfuðburður að honum sakir ættar hans
2. Svo líst mér sem lítill höfuðburður muni mér að þessu barnfóstri.
3. sýsla mér nokkura staðfestu norður þar í sveitum þá er höfuðburður
væri að

'hvað manna' could refer to one person or more:

4. Hvað manna ertu Oddi?
5. Jarl spurði hvað manna þeir væru.

In Old Icelandic, 'væri' is both singular and plural. In Modern
Icelandic, the plural has become 'væru'. Our text with modern
spelling has it as unambiguously grammatically singular (agreeing with
'hvað'), although the reply shows that it has a plural reference:
'Þeir komu aftur og sögðu honum hverjir voru', and Magnus Magnisson
and Hermann Pálsson translate it as plural: "to find out who the
newcomers were."

LN