The guest party always consists of new girls implemented in the gacha/relevant to the current story. It's a way for people to preview the abilities and sprites of the new girls in battle outside of the banner sidebar.

Since the first two or so chapters of events are piss easy anyways, you may as well use it as a chance to preview new girls with the guest party.
Well, if it worked properly anyways. But since we're behind on DMM anyways i guess most people who cared enough would just look at the Japanese wiki.