I have just seen video tutorial for "User Sharing" between Master and Slave, however this seems to be the setup for a brand new website starting from scratch. Is it possible to create a new slave site that shares an already existing database of over 800 users? Or is this option only available when creating sites from scratch?

Yes you can also do it.
In this case, in the template, you have to say that you want to share the users with the "template site" that is your site with the 800 users.

I am following the instructions "How to share the users and extensions between websites".

I implement the following line under:

"list of domain names" : {site_url}/multisites/{site_id}

which then deploys a new site in the selected location with the choosen id:

http://{site_url}/multisites/{site_id}

www.mydomain.com/multisites/mysiteid


But I cannot proceed with installation as I get the following:


www.nextlevelmarket.com/files/multi-sites-error.jpg

Being unable to proceed with installation process. What is wrong?

Check with the "html" source of your page that all the css and all the directories are presents.
If the screenshot is correct, that let suppose that you have "broken" directories or not created directories or something similar.

The only way to know is to check that you have all the directories created and that all the reference performed by your "installation" webpage reference something that exists.

If your screenshot is a "pure" copy of your screen, it seems that css are missing (templates).

Perhaps you have removed the "standard" joomla templates.

hmmm No I have not modified in any way shape or form the files generated by the multi-sits component, neither "broken" directory structures or anything like that.

I don't know what you mean if my screenshot is a "pure" copy of my screen, I simply used "fireshot" for firefox which takes the exact current look of what is showing in firefox and I cropped it unmodified without any image editing or anything like that other than the cropping of the image.

and no, I have not removed the standard joomla templates. What would be the best way to send you a private message?

When I say a pure screenshot, I mean that I see that the "installation" webpage displayed is "broken" as the fields are not at the correct place and that you don't have any "decoration", border, button, ....

This is the reason why I told you to use an HTML debugger like "firebug" for firefox to check the html page and all the links and css to discover which directoy might be missing or that have wrong permission that does not allow to access it.