To create a new anytime collaboration document. Open either Sketch or Flow and go to the main menu (hamburger menu). In the menu you will see that there is an option called share. At this point you have the option of adding a password to protect you shared document. Please note that if you enter a document password you will have to use the password to re-enter the document at a later date. If you do not want to use a password simply hit the 'Share' button.
Once you have clicked the 'Share' button the document will be uploaded into the cloud. In the Shared menu you can see the 'Document ID' which is what is used to gain access to the document.
In the share menu you can also add a password whenever you need to.