Asking :
Are you free ?
Are you busy… ?
Do you have time to…. ?
Shall we …?
Let’s….

Positive responses :
Sure !
I’m free
I don’t have anything planned.
With pleasure !

Negative responses :
Sorry, I have plans
Sorry, I can’t. I’m busy.
I am not available
I have something scheduled
Not really

Organising a time :
How about … ?
Can we meet at … ?
Shall we say… ?