To invite new members to a workspace, click the Invite Users button on a General tab of the workspace you want to invite someone to. Type in user's email and specify their role in the workspace. You can choose several roles at a time.
Before inviting the user, double-check if you are in the right workspace.
Note. Only Super Admins and Workspace Admins can invite new users to a workspace. Super Admins can invite an unlimited number of users, while Workspace Admins are limited by the number of prepaid licenses. Learn more on roles and permissions here.
To change the role of an existing user check out this article.