I'm in the middle of fighting with Group Health to get them to cover my homebirth midwife so my information is fairly current.
It will really depend upon your plan. Check out your certificate of coverage to see if it covers the homebirth. It should be in Section IV, B somewhere if it does. Your homebirth will need to be pre-authorized. The pre-authorization process apparently just consists of your midwife sending in a form or letter to Group Health.
Then, you can go to the Provider Directory and search for midwives. It will not be obvious from the list that is generated if the midwives are only for the hospital or if they do homebirths--you will have to call individually and ask, but some of them actually are homebirth providers, apparently. This is where I've been having difficulties because it is not very intuitive or clear that Group Health has in-network homebirth providers, but they will, generally, will only cover their in-network providers. In fact, if you ask most of the people who work at Group Health, including the midwives who work at the Capital Hill campus, they will tell you that Group Health does not have any homebirth midwives.
Alternatively, if your plan covers homebirths, you can get your PCP or OB/GYN to refer you to an outside homebirth provider. This should get your homebirth covered. I'm in the process of trying this route, but I haven't managed to get it definitively worked out yet so I'm not a hundred percent positive that this is going to work.
(Given that I'm due Sept. 8th, I am desperately trying to get this worked out without having to change providers at this late a date.)
So basically, you first really need to check out your certificate of coverage, and then, good luck finding a provider that is in-network.