General question about finishing.

My fiancee and I will be getting married one week from today, and have a question. In place of a guest book we are having friends and family sign a slab of live edge wood that will be turned into an entry way table. It has already been sanded down and is ready for finish, which will obviously be after everyone has signed. I have a couple questions about this.

• What type of marker should we use that won't smear or be pulled out by sealing the slab?

• What type of sealant should we use to prevent smearing and give us a nice protective coating?