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Before installing the setup file you downloaded, feel free to take a look around ownCloud's web interface, create a new text file, edit it online, share the example files with external users (link), etc.
Regarding sharing, you can share files with other users of your domain, by clicking on the "Share" link next to each file and typing in your colleague's e-mail address. The system will find the address and you must click on it. The file will be shown on the side in "Shared with others" and for the user you shared it with, in "Shared with you". You can also click on the Share link and then mark the check box "Share Link", which is a very useful feature that lets you provide a secure link for anyone to be able to download the file, with options such as password protection and expiration of the link.
Consider watching this short introduction video (5:14 min.): https://www.youtube.com/watch?v=RrAhClhrRAc
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