In the first sentence, the phrase “I rounded up three coworkers for a bicycle trip” is not quite the same as the original Japanese. In the original, it is expressed thusly: 職場後輩と3人で自転車を持ち寄り

職場 – Workplace
後輩 – Junior people
持ち寄り – To gather

So why would I omit these three words in favor of “coworker” and “round up”? Because in English, we don’t make a big deal about the junior or senior status of the people we work with, and I try to make my translations sound natural. Nevertheless, I tried to communicate the same feeling as the original using the phrase “round up”, which implies that Mr. Umi is mostly in charge of the informal gathering and therefore probably more influential or at least a longer-standing employee with a greater social circle.