In the books, Robb Stark legitimizes Jon Snow with several of the northern lords as witnesses, making him heir to Winterfell. Did this ever happen in the show? If so, are any of the witnesses still alive?
Robb does not legitimize Jon in the show like he did in the books.
Or at least, we were not shown it happening...
I know you are asking about the show but, in the books Robb sent Galbart Glover and Maege Mormont to Howland Reed at Greywater Watch. You can see more about what transpired in the books in this answer.
We were not introduced (directly) to any of these characters in the Show, but they were cast and do make appearances.
- Maege Mormont, she first appeared as a background character in Season 1. Lyanna Mormont (the most badass 12 year old) confirmed her death during the War of the Five Kings while Jon was at Bear Island trying to recruit the Northern Houses.
- Galbart Glover, he also first appeared in Season 1 as a background character. Galbart was the Lord of Deepwood Motte, and is presumed dead because we are introduced to Robett Glover when Jon is unsuccessful in his attempt to get the Glovers to join his cause.
- Howland Reed, we only see Howland in the Tower of Joy flashback that Bran has.
We do not know when or how Maege ad Galbart died. They have may still been sent to Greywater Watch and died after they gave the letters to Howland Reed, but that is just speculation at this point. Given that the show is implying things happened off-screen (like Tyrion finding out why Jaime killed the Mad King), I would not be surprised if we are introduced to Howland Reed, that he may pull the letter we are told about in the books out of thin air... however, the best time to have done this would be when Jon is trying to gather forces against the Boltons.
It may not matter (or otherwise complicate) the show's story line if Jon was legitimized because if the R+L=J theory is correct, Jon is bound for a higher calling than Lord of Winterfell.