In ranked PvP you are not supposed to choose all the details of your match, but you can choose how many people (1v1, 2v2 etc.) and whether your team is friends only or randomly assigned. Click the handshake tab (on the right) to either start a team that only your friends can join, or join a team your friend has started. I guess it's possible for someone else on the team leader's friends list to jump onto a friends-only team ahead of one of the invited friends, but hopefully if they are on your friends list they will check first or you can work it out with them.
The system always gets to choose the opposing team based on who is available and what their rankings are. If you want to choose who to fight against, you have to use the practice arena.