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.
The matrix source book (Data Trails) is now out for 5th edition, and no new or better options have been given to technomancers. You can take my suggestions or just have your group house-rule that technomancers can slave devices.
There are several ways for a Technomancer to protect his team's gear.
Data Trails has now been officially released and it is a colossal disappointment for Technomancers. Since this is the supplement that everyone thought would have tons of new goodies for the TM much like Street Grimoire had tons of goodies for magic users, I wouldn't expect any new TM stuff. That means you are stuck with essentially the same options you had before, which I have laid out above. This is as good as it gets, unfortunately. I will update this answer if/when a proper TM supplement is released, but I wouldn't count on it.