To add a collaborator to your site, go to your Client Area, click on your profile icon and select Manage Users. Then, click on the Add New User button and a pop-up window will appear where you can select the collaborator option.
Next, select the website which you want to give the collaborator access to. Add their name and email address in the next fields too, and click Add. The person will receive an email invitation to create his collaborator profile with SiteGround.