Until my 17 yo son just got his own phone, all my friends understood that me and my kids share a phone. Now it's just DD and me. Also the understanding in our house is anyone living in the house is free to look at anything on anyone's phones (in our house)*. Our friends are aware of this, no nothing of a personal nature is sent by text. My DD will ask me how I want to respond to a text intended for me if she's using the phone but neither of us take pains to identify who's actually reading, interpreting and answering the text. I will, on occasion, answer a text for my daughter if she's not around (it is, at the end of the day, MY phone) but I always let the recipient know it's me and not DD.
*Safety precaution - our kids have a relative expectation of privacy. We have the passwords to their emails, forums, the one social network they're allowed to participate in and they understand that as long as they are living under our roof, we retain the right to check the texts and calls on phones that they pay for. My son is still a minor and has minor friends and we went over the risks of having a cellphone (sexting). They also have access to anything on our phones.