I was on here looking to see if anyone else had an answer to my question about the POST activities endpoint I just sent up and noticed your question. I assume you already figured this out since it was asked in January, but maybe my response will help others looking for user ID's.
As a side note: I'd assume there is an endpoint in the API to list users, but if you don't have a ton of people, you can quickly go into Settings -> User Management -> Click a user name, and in the top navigation, you'll see the user name and in parentheses their ID (#123456)
I hope this helps you or someone else.