On the master, set ROS_IP to its IP address (since other computers probably can't resolve its name on the network) and run roscore:
export ROS_IP=<ip address> roscore
roscore will print out ROS_MASTER_URI, which needs to be set on all other machines.
Using the ROS_MASTER_URI displayed on master:
export ROS_MASTER_URI=http://<ip address>:11311/
You will probably need to set ROS_IP on the other machines with their own IP addresses also, if they are publishing anything. In your ros_workspace/setup.bash, you can read the computer's IP address automatically every time you open a new shell:
export ROS_IP=`ifconfig wlan0 | grep 'inet addr:' | sed -r 's/^.*inet addr:(\S*).*$/\1/'`