Central joomla site and database running multiple sites

We have a requirement for a Joomla solution to the following. A client has a Joomla community site, this is proving popular but in addition to this he has several other community sites (same type of community) and would prefer to have everything "under one roof" so we have suggested a single database running multiple sites, each site having the same core members but each site targeting different types of member - the site is adult related. If you have any experience of building such a site structure - a single database running multiple sites - please bid and we can discuss further. We would prefer to base the sites on Joomla for our convenience and cost, but if you have an alternative, so long as the price is right we can discuss. This is an intial project enuqiry at this stage, however in conjunction with our client we will discuss and select a provider who will be employed at the agreed price to complete the work. Payment will be via escrow, with agreed milestones prior to commencement. Questions via PMB please.