You can find the "add users" section via two routes:
Anywhere in your account: Navigate to the profile button in the top right of your screen and click.
Next, click on Organization Settings.
OR
In A Workspace: Navigate to the side bar on the left hand side of your screen and click on Project Settings.
Next, click on the button titled Edit Organization Details at the top of the screen and you'll get to the Settings page.
Add Members
To add a member to your project, click the button titled Add Members. You'll need to add their name and email, along with selecting their permissions and the workspaces they are allowed to access.
Workspaces
Workspaces the highest level of organization for your account. It essentially splits your account into research campaigns, within which you create projects, which in turn house individual studies.
In the navbar on the left side of the screen, you'll see a tab titled Workspaces.
Here, you'll be able to edit everything you need to about your workspaces, depending on your permissions. If you want to edit the details of a workspace, simply click on a workspace and you'll be able to modify membership and permissions. Read our article on understanding user types and permissions to learn more.