I assume, that you are using a serial bluetooth device, such as HC-06 or HC-05. As I know, even the master module HC-05 can only connect to one client at a time (which makes sense, since the serial connection on the other side is the same)(despite the fact, that bluetooth in principle allows handling multiple clients at a time, though it's a rather complex task). This means, that you would have to use one serial bluetooth module for each phone, you want to connect at the same time.
When using more than one bluetooth module, you have to connect them to the Arduino over serial, but only one device can use the serial connection at a time. Depending on the number of phones, you want to connect, you can use either the SoftwareSerial library to emulate more than one serial connection (so every connection can be identified by its own SoftwareSerial object), or you have to multiplex your data lines between the different serial bluetooth devices (so that you control the serial connection to the devices with for example setting digital pins). For the last mentioned possibilities, I'm not sure about the property limits in the serial connection. Maybe you can multiplex just only with a few transistors.
If you want not only distinguish between the different connections but directly between different phones (for example to remember special settings for only your phone, not the others) you would have to send an identification over bluetooth to the Arduino (for example the MAC adress of the phone).
For more help, you should state you question in a better way and telling more details
