With SharePoint Server 2013 and Office365/SharePoint Online you can create and use Community Sites.

General Information: http://office.microsoft.com/en-us/create-a-community-HA102891678.aspx

New members can join and a moderator should watch the discussions. Important things when the moderator or administrator wants to get alerted, might be:

  • New Members joined
  • Someone reported offensive content

To see the reports and manage abuse-reports the moderator/administrator has to go to the site to check it actively. There is no automatic “reporting service” by sending an email to moderator or administrator, so what else you could do?

Using Alerts is still possible but in case you do not want to “break” permissions on hidden lists like AbuseReports, a good way to go is using workflows.

Example:

You enabled the “Reporting of offensive content” for your community discussion site:

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Your community site URL starts with:
http://contoso.com/OurDiscussion/_layouts/15/start.aspx#/SitePages/Community%20Home.aspx

Remove _layouts… and the rest of the URL, add Lists/AbuseReports.AllItems.aspx

It may look like this:
http://contoso.com/OurDiscussion/Lists/AbuseReports/AllItems.aspx

Now you can start to add a workflow:
Create a workflow using SharePoint Designer:

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My suggestion is to use a SharePoint 2013 Workflow:

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Just an email and then End the workflow.

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Decide how you want to start the workflow:

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You need to publish it:

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This is just one example for a lot of different ways you can customize SharePoint to fulfill your needs.