If in the photo people faces are distinctly seen models consent “Model Realease” is necessary.
You will find it practically in all the pages of the microstock. An example of “model release” :
If the people faces are seen very vaguely (in the background, indistinct zone or similar) or the people backs are seen – the consent is not necessary. For many people it seems to be too much fiddling with collecting signatures. One should not be afraid of that moment.
Surely you will not run after an accidental passer-by in the street with a fountain pen and sheet of paper, however, in the circle of friends or relatives there should not be a big problem to get a signature. While requesting for a signature promise beer or wine “if everything is a success” and young people especially girls I think will not avoid the name of a model. In most cases a promise to send a perfectly made photo should be sufficient.