I'm pretty sure the funeral's a private event.
I can't really think of much that's more classless then showing up to a funeral uninvited.
It depends entirely on the circumstances.
I've been to funerals where there were a dozen people, and showing up would have been very, very weird, and I've been to funerals where there were hundreds of people, many of whom I didn't recognize at all despite being very close with the deceased, and nobody batted an eye.
I'd assume that this is private to some extent, as it'd be an opportunity to meet famous people, and I'm sadly sure that there are some folks who would take that chance for the story or to try to get an autograph or some stupid thing like that. However, I wouldn't be surprised if some friends of Marty's, like players from other teams, showed up and it wasn't a big deal. I'd imagine they don't want complete strangers there pestering anyone as they mourn, but they'd likely be open to people that the family knows whether they were specifically invited or not.
Just taking some educated guesses based on the funerals I've been to. Funerals are for the living. If someone can help a mourning family feel a little better, I'm confident that they'd be welcome.