Unfortunately you can only play against your friends in 8 ball pool if both have the same version of the game.
If a new update is released, sometimes it's not released for everyone at the same time. Due to this, what can happen is that some of your friends will have the latest version and some won't, making it impossible to play against friends that have a different version.
The only thing you, and your friends, can do is wait patiently for the new update to be available in the official stores to everyone.
We are sorry if this has caused you any inconvenience.