Go To "Security" tab in MongoDB Atlas cluster, from there you can "edit" or "Add a new User".
@Alex Blex, Let's start from Beginning, You want to create the 'user' admin. For what purpose you want to add the user. At "MongoDB Atlas Cluster" there is lists of "Role" Available like
Roles/Privileges
backup
clusterMonitor
dbAdmin
dbAdminAnyDatabase
enableSharding
read
readAnyDatabase
read
readAnyDatabase
readWrite
readWriteAnyDatabase
Suppose that you want to create a new "User" with "readWriteAnyDatabase" Role.
Step 1 : First Login to "MongoDB Atlas Cluster" through your User ID & Password Credential.

Step 2 : After login at first time you shall find the two Tab
- Overview
- Security
Click on the "Security" tab.

Step 3 : On the Right side above corner you shall find out the button "ADD NEW USER" click on that.
Step 4 : After click on "ADD NEW USER" , you shall find the screen as mention below.

Step 5 : Noted Down here, at this screen you decide what you want to do. I mean to say that what kind of User role you want to assigned.
If you shall not select "Show Advanced Options" then three tab
- Atlas admin
- Read and write any database
- Only read any database
Will be available. And according to your requirement you can choose the option and their respective "User ID" and Password.
eg; Like "Atlas Login"

& "Read and write any database"

Step 6: If you shall click on "Advanced Roles/Privileges" the you shall find out
option like

And the "Advanced Roles/Privileges"

Select the "Role" according to your requirement and set their "User ID" and "Password".
I hope so that it will help out to you.
After chat with @Alex i am creating the "Alex Blex" User with
"dbAdmin" role for "AlexBlexDatabase".
See the screen shot below
