You can upload files to your personal repository or to groups you are an organizer in.
Go to the Actions tab of the Start Organizing menu, and click Files. Then, select the group you're working from.
When viewing those repositories, click the "Add New File" button in the upper right. Choose the file from your device's hard drive you want to upload, fill out your file's name, and enter an optional description.
Click the "Add File" button to finish and upload your file. Your new file will appear at the bottom of the list in the current folder your are viewing.