A decker would simply slave important devices to his deck and use his deck's Firewall rating (plus any running programs) to defend his team from Matrix attack, but a Technomancer's living persona specifically cannot be a master or slave. Should the whole team just get the best commlinks they can afford and protect their own devices, or can the Technomancer bring his persona to bear against Matrix attacks against his team even if he hasn't already spotted and marked an incoming threat?
A lot of people think that the rule that technomancers can't slave devices is a little weird. But technomancers do interact with the matrix in a different way. And, because it's different, you have to figure out a different solution to the problem.
One solution to this problem is to always be running in the Matrix and making Matrix perception tests to check for problems on your team's devices. Another is to thread a machine sprite, stick it in a commlink, slave everyone to your sprited commlink, and then have it warn you when someone's trying to hack into your devices.
My guess? When the matrix source book comes out for 5th edition, they'll be giving technomancers better options. In the mean time, you can take my suggestions or just have your group house-rule that technomancers can slave devices.