As the office manager, it's their responsibility to ensure the office space is used optimally and that everyone has a good space to work, stocked with the supplies they need. If they first go around and ask every employee what they would think then they are having someone else take their responsibility for them.
If you have concerns that the addition of an extra desk will detract from your ability to do your work, you can raise this with the office manager politely, but until you've actually experienced working in the new set-up for a while, you may want to hold off on that. There is a very large chance that you will be seen as 'one of those guys who is afraid of any change', which can lead to problems down the road.
Also remember to only speak for yourself, don't speak for your colleagues. You may think you have a stronger position if you speak for "the team" but at the same time, when you make a complaint you are raising a negative flag, which will reflect badly on you. Even if they agree with you, it's for your colleagues to decide if they feel strongly enough about the issue to raise this negative flag as well.