How does surrogate motherhood work?

I have two friends, a gay couple who I've known for years. They want to have kids that are their own biologically rather than adopting, but they want the mother to be someone they know, so they asked me. How would that work? Would my name be on the child's birth certificate as their mother, would the non-biological father have to technically 'adopt' the child...? It's confusing! I know what our arrangement personally would be - the child would know who their biological mother is but I wouldn't be involved in any decisions about their parenting or have them live with me at all. I have my own husband and we're planning on having kids of their own. But what I'm asking is, how does this work legally?