On your desktop, you will find the Zoom icon! Double-click to open!
From there, just sign-in using your Zoom credentials (Note: These are different from your general login credentials)
Once logged in, you will immediately be taken to the "Team Chat" tab. From here, you can create new chats or channels with the + button.
Under the "Contacts" tab, you will find all the respective departments where you can expand the lists and chat with any member of any department (as long as they're available of course!).
Helpful Hint - You can click the "Star" next to their name to make them a favorite (starred) contact. You can do this if you frequently chat with this person.