Reservations booked at the same together are grouped together whenever possible. The numbering system on some trains may seem like you're not sitting next to someone in your group, but you probably are.
Note that ItaliaRail is unable to change the seats assigned by Trenitalia. You can, however, attempt to make changes to the seating assignments at the ticket counter at the station, provided there are open seats to which you can be moved.
If you're traveling on a train that does not require reservations, you're at the mercy of whatever seats are available when you board.
Learn more about seating configurations on trains.