When you form a play group and start playing a game, the DM often gets some kind of authority given to it by the game. However, the DM does not (and should not) do the playgroup – the group does. They have as much (but no more) say in group-influencing decisions as any other member of the group.
Just because someone is DM does not automatically give him the right to add (or remove) players without consulting the group, as he does not have the right to know the place and time of the night, the type and the time Presence arbitrarily to change from eating at the table or other from the game details.
Now you could give the DM this authority, but that would have to be something that leaves the group of DMs, and not something that is automatically given to them without discussion. If you did not explicitly have this discussion, you can not and should not make that (or any) decision for the group.
You should talk to the DM as soon as possible, not just for the sake of your current group size, but also because a DM who thinks she can add players at will, without asking, may have a completely different view of her role at the table has you and the rest of the group.
I was in the position of the DM and the player both as a person who suggested someone new to the table, and as someone who was worried about another suggestion to add a player. So I speak with experience when I say that it is the right way to discuss this as a group (and that not talking to the group means trouble).
We have had several times when the discussion with the group has picked up on some serious issues. For example, the DM once wanted to invite a player who not only (without knowing it) was extremely offensive to several other members of the table, but also had logistical issues that would have a negative impact on everyone. In the end we talked about it as a group and decided not to add the person. I strongly believe that adding this person through the DM without consulting us would not only have caused a great breach of trust, but also killed our group.
We have also had much more positive experiences where everyone talked about it and decided to let someone else in.
Big groups are not easy
Quite apart from the issue of who decides who plays at the table, the number of players can dramatically influence the gameplay. Larger groups are exacerbating many of the issues little groups are already facing and can bring some new ones to the table. If the players and DM do not know this can (but not necessarily) a recipe for Unfun happening.
I played with more and less. There is no perfect number for each table, but you have a right and a foundation on which you can base your concerns about adding people, and you should definitely bring them to your DM.