There could be a couple reasons why.
First, ensure you've purchased a premium SideDrawer account. Only a premium account allows users to collaborate with others.
Your collaborator must have their own SideDrawer account as well. It can be a free account, but they'll only be allowed to view your records and they won't be able to share their records or edit yours.
Also check that your collaborator has been given permission to access your documents by opening the record and scrolling to "Collaborators & Permissions".
For more information about changing permission settings, please read the article on "Understanding and changing permission settings" here.
If you have no collaborators it's very easy to add them. Simply click "Add your first collaborator" or the + sign to add an additional collaborator to that record.
Then fill out the required information and click "Add new collaborator".