Tell me please which sentence sounds grammatically correct.
What do you say we meet up for a cup of coffee sometime.
What do you say if we meet up for a cup of coffee sometime.
What I am trying to say is I am asking if the person would like to meet up for a cup of coffee.
The first sentence is idiomatic:
✔ What do you say we meet up for a cup of coffee sometime?
The second sentence is not. In order to add if, the verb tenses also need to be changed:
✘ What do you say if we meet up for a cup of coffee sometime?
✔ What would you say if we met up for a cup of coffee sometime?
"What do you say we X?" is just one of many idiomatic ways of saying "Do you want to X?" (as in, do X together). Literally, I guess it means, "what do you say in response to the suggestion that we X?"
You could also tag it on the end of a suggestion like so: "I'd like to meet for a cup of coffee later. What do you say?"
I wouldn't analyze it further than that.
There are lots of idiomatic ways of saying this, for example:
Let's meet up for a cup of coffee sometime
Would you fancy a cup of coffee at a time that suits you?
How about a cup of coffee once you're finished work?
I'd love a chat over a cup of coffee at your leisure
May I invite you to a cup of coffee when you're free?
And so on....
