When it comes to creating a new user for any WordPress website the easiest way I found is using WP CLI command line tool. Luckily this is already installed on WP Engine. This way creating a new user is just running a a series of commands.
Bit first you need to SSH into the right WP Engine environment. For this yo can follow the steps I outlined in Using WP CLI on WP Engine.
Once that is set up go ahead and open a terminal on your computer and do the following.
Connect to the environment via ssh, simply run the following
Once connected to the environment
wp user create USERNAME YOUR@EMAIL.COM --role=administrator
This will generate a random strong password for the just created user and display it your console. Note that a USERID (a number was returned as well when the user was created)
If you wish to change the users password you can use the user id number that was returned in the create user command and run the following.
wp user update USERID --user_pass=<YOUROWNSTRONGPASS>
I hope this helped!