Several apps running

Would it be possible to connect one Arduino running a blynk application with several android devices? F.x. an home alarm system, where all family members can receive alerts and also turn the home alarm system on or off by a button on the app?

I believe the only way to do this is by login with the same account in all android devices…

Right now you could do so using same login. But next Android update will have sharing functionality (only for android platform for now). So it will be much simpler.