Granular roles and data-level permissions are only available on Growth or Custom plans. On Starter, everyone you add is a Member with access to all data.
Roles
Only owners and admins can invite users, change roles, create Groups, and manage workspace settings. Members cannot create layers or see layers they do not have permission to access.
To add people and assign roles, use the Members area in workspace settings. For the invite flow, see Invite team members.