"I just don't understand how SETI or any other group could define what it means to be human"

Sounds a bit silly.
But how a message of welcome?
Build a Mcdonald's for aliens. Sure we don't know what they like to eat. And probably no alien will show up. But this in itself would explain their form of human culture better than anything else.

So they would need space which provides containment against human/earth infecting alien and alien infecting human/earth.
And some place to park the flying saucer.
Internet connection might be good.
And food could be wide variety of stuff which is sterile and mostly carbon based.