Go to Settings -> Collaborators to add collaborators to your dataset.
Collaborators can have 3 roles: labeler, reviewer and administrator. Each role has different permissions:
Manage samples (view, upload, delete)
Manage releases (view, create, delete)
Labelers and reviewers can only see the "Overview" tab of a dataset, where they can click the "Start labeling" and "Start reviewing" buttons depending on their role. Note that they cannot browse the samples on the Samples tab.