Basically, all i remember is that they're not actually invisible. But people don't notice they're there at all.
I think I remember an old Star Trek episode like this. I think they were moving really fast and when they were around the people in regular time, the regular time people could hear a buzzing or something like that. Do you think it could be an old Star Trek episode?
I don't think this is what the OP was looking for, but there was an episode of
"Space Academy" where at least one person somehow became invisible, I think because they were moving really fast and just couldn't be seen. (Or was it the sequel
"Jason of Star Command"?)
More recently,
"Stargate SG-1" had a few episodes like this:
In episode #3.21
"Crystal Skull", Daniel Jackson becomes invisible after looking into the eyes of a crystal skull.
In episode #4.3
"Upgrades", the team wear special alien armbands that give them greatly increased speed and strength; when moving fast they are basically just blurs that "normal" people can't even react to.
In episode #9.18
"Arthur's Mantle", Sam and Colonel Mitchell get "translated" into an alternate dimension where no one can see them but they're still there.
In episode #10.12
"Line in the Sand" Sam uses the same technology to hide a building and the people in it.
It's not likely, but if
any of these are what the OP was looking for, it is probably SG-1 episode 9.18.