There are three types of user roles:
Segment-user, not admin
Account-user, not admin
As a Segment user, you can only see the data on the segment you are invited to. You cannot invite other users or create segments. A Segment user is invited to the dashboard via the menu on the left by clicking on "add member". If you are a Segment user admin, then you can invite new segment users, and create undersegments within the segment you have access to.
As Account-user (whether you are an admin or not), you can view the data on all segments of the account. If you are Account-user and admin, you can invite other users and create segments. If you do not have admin access, you cannot invite new users.
You invite an Account user in the dashboard by clicking at the top right of the user icon. Click Settings, then Invite.
When you click on the invite button, fill in the email address and decide whether the user should be able to invite other users or not (This is done by choosing whether the user should be admin or not).