Say you’re at an Orientation Week event and you’ve been talking to a group of people. They all announce they’re going to check out a certain place once the event is over. They seem friendly enough to you, but they didn’t specifically invite you along. In cases like this just go along as well or ask if it’s cool if you can come too. It’s not like this is some long-established social circle that you’re intruding on. Nothing has formed yet. It’s cool to just tag along with these proto groups and see where things go.
You may also find yourself in a situation where you know there are certain freshmen events going on later that day, but for whatever reason you find yourself alone in your room with no one to go with. That first week is chaotic and things like this can easily happen. Just go to the event by yourself and then try to meet or run into people once you’re there. Again, everything is up in the air so this is fine. Even the students who came in groups will be meeting lots of new people there, so it’s fine if you’re doing the exact same thing, but just happened to show up by yourself.