Admin can add team members to their business account in Standard Plan. To add users, you will need sufficient licenses in the business account.

Step 1:

Go to Settings > Manage Users

Step 2:

Click on the '+ User' icon on the screen

Step 3:

Enter the details of the user you want to add to your business account

Step 4:

The user is now added to the account. The invitees will get an email with the account id of the business account. They can now sign up and add to the business account by entering the account id. 

The process is listed in the article here.