Is your assignment to assign where they are going to be sitting? Or just to make sure there is space available? Two different functions, though complementary.
Who keeps track of what spaces belong to who? If there is someone with that task, go to them and figure out what is available. If there is nobody, maybe that is a job you should assign yourself (with the permission of whoever is authorized to give that permission). Then you will need to draw yourself a map of all spaces and who they belong to. If they aren't labeled, go talk to people. Find out who they work for and with. Managers and teams. Add that information to your map; you'll need it later.
Once you know what is available, the next step is to find out who is rolling down the pipeline. Do you have a list of people who will need seating? Or is your task a general "you are in charge of making sure new hires have space". If the first, easy enough to figure out who each new hire will be interacting with and assign them a place as close as possible to their team.
If you don't know, your job is going to take a lot more effort. You will either need to find a way to be sure nobody is assigning spaces but you, or you will need to keep checking your "empty" spaces to make sure they are still available. Again, each time a new person is hired you need to find out who their team is and assign them as close together as possible. The new folks need access to more experienced teammates more than the old-timers.
You might be able to get a few extra brownie points by mapping out all the seating for all the teams and if they aren't together you might suggest some space-swapping to move teams closer together. Be sensitive to the fact that some spots are more "choice" than others; don't ask a guy with a window seat if he wants to swap with the kid who they put in the tiny cube next to the bathroom :)
If people aren't open to that, nothing lost but if they are you might be able to impress more than just your boss.