Most likely you do not have the correct set of permissions or you are not using your SciNote Premium.
The Members, Security, and System log tabs are only available for Premium customers and can only be seen by the Organization administrator.
The Members tab allows you to manage your workforce within SciNote. It is where you would invite your colleagues and remove those that leave the company. With this in mind, it is limited to the Organization administrator. If you should have access to it, please, contact your Organization administrator so he can upgrade your set of permissions.
You can read more about what the Organization admin can do in the following article.
To learn more about how to manage your SciNote, please see the video tutorial below.