I like to account for 1/3 to 1/2 pound of meat per person. But, I come from a family of big eaters.
Since another person is bringing ground turkey, I would account for 1/4 lb of meat per person and thus bring 5lb.
Onlyme, I would say 1lb meat makes 12 single tacos, and not overly full either. In my extended family, that would be enough for 3 people, max.