When it comes to using Outlook with G Suite, your end users have two options.

If the end user wants to use IMAP, they will utilize the default Gmail IMAP settings found here: https://support.google.com/mail/answer/7126229?visit_id=1-636554425529107293-2915571729&rd=1

This includes the port settings and security information. 

Additionally, your administrator needs to ensure IMAP access is enabled for user accounts

Instructions: https://support.google.com/a/answer/105694?hl=en

On the other hand, your end user will get more usage out of Outlook by using G Suite Sync for Microsoft Outlook.

Overview: https://support.google.com/a/users/answer/153866?hl=en

Essentially, Google has incorporated the protocol Outlook uses to better align G Suite with those Outlook features. It will require work on the end of your administrator to ensure GSSMO access is enabled for your domain. From there, your end user would download said tool from Google (link below) and follow the configuration instructions in the below link. This is a better tool for working with Outlook than IMAP.

Admin Setup Instructions: https://support.google.com/a/answer/148484#gappsreq

Step by step setup instructions for end users: https://support.google.com/a/users/answer/171710

Download: https://support.google.com/a/users/answer/153871