They introduced Shizuku and Umiko much earlier in the anime, but I suppose that is pretty standard in adaptations.
According to a director interview, it's because they're popular characters, so they're having them be active characters from earlier on to please fans.
Oh crap, the anime's out already?! It's not even up on Nyaa. Woo! \o/
It'll never be on nyaa because CIRCUMSTANCES.
First episode is about as good as can be reasonably hoped for. Changes include some new material, yuri elements being played up, and a touch more fanservice
What confuses me though is that the OP heavily features Hajime and Yun
Again, from the director interview, they want to end the series at "a really good ending point" aka probably the end of volume 2, meaning they didn't have enough material to fill it out. So they asked the author to write them an extra episode worth of material (the manga-version of that meeting of Umiko and Hazuki was published in the interview). He also stated that with the anime-original material he wanted to try and show more scenes outside of the office, hence the "my first kouhai" conversation taking place when they go out for lunch instead of at their desks.
He also stated that his favorite character is Hajime. What effect that may or may not have had on the OP, I can't say.