What do I put for an unmarried couple?

If it was a friend of mine I put their last name in as household, then put for example "Smith,Boyfriend" and if it was one of my FH friends I would put his last name as the household and I put "Smith,Girlfriend" FH has a friend that has a new girlfriend every month so I figured if I did that then I wouldn't have to change it later and when time comes to send out invites, I am going to put something like "there are 2 seats reserved in your name"

I listed the house hold by the person that I knew best. If I was friends with Jane Doe and she was living with Jack Black, I put the household under Doe, Jane and listed them both in the household. If you wanted to go by more traditional methods then list the 'man of the house' as the household name and then both in the house hold. As far as the invitations go you'd list both names when addressing the invitation

Mr. Jack Black &

Miss. Jane Doe

I'd do as Konichiwa said.. I'd address the invite as

Mr. Smith &

Ms. Jones

or with first names as stated..

Mr. Michael Smith &

Ms. Katrina Jones

For married couples: Mr. and Mrs. John Smith

Unmarried couples: Mr. John Smith

Ms. Kate Jones

Write out both their names, but on separate lines.

I would go w/Mayo on this... it's just easier and doesn't put any of them in an awkward situation with their significant other as well.