1.) After logging into Apple App Store Connect, click on Users and Access.
2.) Anyone you wish to add to your team needs to have an Apple ID, but they do not need a separate Developer account. To add users, click on the blue People tab at the top of the Users and Access screen.
3.) Click on the blue plus sign
4.) Fill in the person's name (can be anything) and the email that they use with their Apple ID.
5.) Assign a role(s) to your new team member. You can click See Permissions to compare the different role types.
6.) Optionally, you can let them create reports and apps as well. This may be helpful if you run an app-making agency where people are working on different projects.
7.) Lastly, use the dropdown to select which existing apps of yours the team member can have access to. You can select as many as you need to. This is optional and if you chose Admin as the user role, they will have access to all apps by default.
8.) When you're finished, just click Invite in the lower right corner of the popup. The invitation is send and will expire after a couple of days. It's usually best to make your team aware of the invitation beforehand.