There's no "People" tab, neither "collaborators" section

[Sergey Dolin]

This question is in reference to Atlassian Documentation: Managing collaborators

Fix the article describing the 

Create the article describing how to setup cloud instance for both JIRA and service desk.

[Nic Brough -Adaptavist-]

There is if you're using Service Desk 2.5, which is what that documentation is for.

It's changed in 3 (and hence on Cloud) - try reading the docs for the latest version.