Once you launch the app and swipe into the PartyUp screen, it will automatically scan for any available speakers and list them at the bottom of the screen. You can then simply drag and drop them onto the main area, where they'll display as either a full speaker icon (if just doing stereo) or small discs for three or more speakers. They'll display with a loading icon in the center until they're fully connected, or occasionally drop back into the found speakers list if the pairing didn't work the first time.
The design and interface is much improved over previous iterations, although it's not a drastic overhaul. Connectivity and pairing is generally better too, although we're at the mercy of Bluetooth, so there's still a few potential hurdles. I was initially shown the feature on aniPhone 6 and it seemed to work near-flawlessly, finding 10 nearby speakers quickly and adding them to the group, even if they were powered off at the time.
My subsequent tests were done using a Nexus 5X runningAndroid 7.0, with mixed success. Sometimes I had to toggle Bluetooth on and off a few times to get things going. This happened a few times during my tests and isn't a particularly new occurrence, at least with handset I've been using. Once the app connected to the main speaker and found the other devices nearby, things proceeded smoothly, for the most part. As you can see in video below, it took some time to load and a few tries to get them all connected.