It’s best to have them add you as a user to their accounts instead of sharing their usernames and passwords with you. That way all your changes will be tracked under your account so you can’t be accused of something someone else did. Also, if you were to ever part ways, they can easily shut off your access to their accounts, thus protecting you from any accusations down the road.
The files for a web page (such as the text and images) need to be stored somewhere. Then when you try to access the page in your browser, those files will be loaded so that you can see the page. It’s similar to how you have folders on your desktop. When you open a folder, boom, the files are there for you to see. A website hosting service does something similar for you. It serves the files needed to view a website from a server.
Log into your GoDaddy account
Click on the “Account Settings” tab and select “Delegate Access”
Here you can invite someone to access your GoDaddy hosting and domain or request access.
Most likely you will need want to choose the “Products, Domains, & Purchase” access level. Why? So that you can configure hosting, the domain name and if the client approves, make additional purchases (such as website backup services).
You can learn more about the access levels here: https://www.godaddy.com/help/delegate-access-levels-of-permission-12374
Log into your WP Engine account
Click on the “Users” tab and select “Add User”
For the “Account Access” level you’ll probably want to choose “Owner”. Why? So that you can add or remove users as needed and delete an environment if needed.
You can learn more about the access levels here: https://wpengine.com/support/user-portal-user-roles-differences/
Shared hosting on SiteGround only allows you to have one username and password for all sites in your account. Unfortunately, the only way to allow access to SiteGround is to get these login credentials from your client but once you log in, you will be able to see and access all of the sites in their account. The login page for SiteGround is here: https://login.siteground.com/login.
If you have dedicate hosting with SiteGround, then you should be able to be given cPanel access individually for each site that you need to access.