how to count how many users and how many comments to every post in mysql? : here is my comments table

    id(int) user_id(int) post_id(int) text(char)

I want to calculate how many users has comments to every post
and how many comments do every post have. 

I've already done the latter one as following:

    select count(*) from comments group by post_id;

but how about the first one? any ideas?