There is a number for each group, you can find that by hovering over them in the ACP and see it in the bottom left hand corner. I believe this would work, $xf.visitor.isMemberOf(10, 2, 1, 4, 5)
I tried that and it doesn't work for the user I'm testing and I think it's because that statement means they would only see it if they are part of ALL of those groups (AND, not OR).
Is there a way to get it so it's 6 or 7 or 8 or 14?