I have been asked to build a Multisite setup using Joomla.

The best way to explain would be like this;

Main site www.mainsitecounrty to carry the site template and main database

Subsite www.city1/mainsite to carry a forum-banners-blogs using the main site template and main database + a city1 database

Subsite www.city2/mainsite to carry a forum-banners-blogs using the main site template and main database + a city2 database

Subsite www.city3/mainsite to carry a forum-banners-blogs using the main site template and main database + a city3 database

There could be up to 10 sub domains with the Main site www.mainsitecounrty having links to all the latest input from the sub city domains.

As far as I can work out they want people to be able to read ALL the info but only post in a city that they have registered in

Can this be done?
If not is there a way that something similar could be built?

Cheers

First, a joomla website is connected to one DB and not on several DB.
What JMS can do is in the case of the "Article Sharing for JMS" or the
"Multi Sites Contact" is read the article coming from another website (DB).

So when you mentioned that a website is the sum (or concatenation) of
several DB, I am not sure to understand what you mean.

What must be refined is what you mean by read all info.
The word "info" is too generic.
To read all the info, I suppose that you want to be able sharing the info.
JMS can only share the content of a limited number of extensions.
So share ALL is not possible except when you decide to share the whole
content - The problem is that this whole content can be modified by the
users - Things that you mentioned as not OK.

Based on what I understand, you want to have a city website that have
limited content and be able to read other content from other website.
Such kind of thing is possible with the "Article Sharing for JMS" but is
only OK for the joomla article and not for the other content.
You can do that also with the "Multi Site Contact" that can be read from
another website but I suspect that you would like to have specific contact
for each city.

So I think that JMS can do that partially as this depends on the extension
content.
This is not possible with all the extension content.

You have to verify the exhaustive list of extension for which you want to
share the content.
See also the list of extension that can have their content shared.
When you share the content of an extension, it is available for Read/Write
but with the Joomla ACL, it is possible to restrict the access.
www.jms2win.com/en/list-of-extensions?vi...id=269&Itemid=53