The preposition "um" and a number... indicates always the hour, as far as I know, when you speak no need of saying "Uhr". It is pretty common not to say "Uhr". This course is full of sentences that are even more informal of this one, I don't know if it is a good or a bad thing.
It is the actual meaning, yes.
Edit: Although I have to add, I'm not too happy about duo leaving out 'Uhr' and using a twelve-hour clock. I consider this sentence very colloquial. In most cases I would suggest 'um dreizehn Uhr' for 'um eins'. Might be my upbringing or socialisation only, though.
Six groups arriving at one o'clock.
That's what I would say. :-)
I was close though. Constructions with "to arrive" were not accepted. :-)