Username or EmailPassword
In the US we totally pay through the nose for text messages. I pay USD 25 a month for an unlimited text plan for the family. Using iMessage I can go down to a basic plan which costs only USD 10 so this one app would be a huge savings.
(Needless to say, this only applies if your family members all have an iPhone which is probably the whole reason for this app, to lock in the whole family into iThings.)