The chi-rho, as Tom shared, is a symbol representing the name of Christ. Christians in the early church developed it as a way of representing in particular the victory they had through Jesus's death on the cross. Later, the symbol would be adopted by the first Roman emperor to become a Christian, Constantine. Constantine adapted the chi-rho for use in his battle standards, to send his soldiers into the fight marked with a symbol of victory through Christ. The symbol would be used by a few later Roman emperors as well.
Hopefully all of that should explain why our friends in Disciple chose it: it represents Jesus, His cross, His resurrection, and the victory we have through Him.