Adding documents to your profile is described in https://explore.researchgate.net/display/support/How+to+add+research . Editing the details is explained in https://explore.researchgate.net/display/support/Reviewing%2C+featuring%2C+and+editing+your+research . For how to change the list of authors see https://explore.researchgate.net/display/support/Authorship .

IMHO: Select "Skip" and create the document page without adding the full text file. After creating the empty page click "add full text" link and add full text file and coauthor.

Right

You have to invite the coauthor trough the link Researchgate provides. The response of the coauthor will lead to the uploading of the work.

When you will access to your profile in the Research Gate.net then on the right top corner you have to click on the menu Add to New Document. Then various options will be there and you will choose one appropriate option. In case of Article you click on that and next steps can be easily understood. So do not worry and just add your article