We have setup a Salesforce idp and we now want our community users (lic: Customer Community Login) to be able to login through the idp to our community and our 3rd party applications. How do we do this? - Answers - Salesforce Trailblazer Community
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Michael NordlingMichael Nordling 

We have setup a Salesforce idp and we now want our community users (lic: Customer Community Login) to be able to login through the idp to our community and our 3rd party applications. How do we do this?

We have already written code for this and we are able to login through the SF idp using a Standard user, but not a community user, we are using SAML v2. 
Best Answer chosen by Michael Nordling
Michael NordlingMichael Nordling
We were able to get this to work by changing the standard Identity provider url to the community url, this is an un-documented feature as far as we can tell, Salesforce support also confirmed this. 

So post the SAML request to https://[communityname].force.com/idp/endpoint/HttpPost instead of e.g. https://[custom].my.salesforce.com/idp/endpoint/HttpPost then a community user can be authenticated. 

Hopefully this will help someone else. 

All Answers

Jeff MayJeff May
Remember that the Community has a different URL than the Production org.  Here is a link to the Communities Implementation Guide which has full documentation about SSO security configuration:   http://na11.salesforce.com/help/doc/en/salesforce_communities_implementation.pdf
Michael NordlingMichael Nordling
We were able to get this to work by changing the standard Identity provider url to the community url, this is an un-documented feature as far as we can tell, Salesforce support also confirmed this. 

So post the SAML request to https://[communityname].force.com/idp/endpoint/HttpPost instead of e.g. https://[custom].my.salesforce.com/idp/endpoint/HttpPost then a community user can be authenticated. 

Hopefully this will help someone else. 
This was selected as the best answer