There are a couple of resources available for IBM Connections developers. Below is a list of key resources that I used to implement this sample.
The redbook describes how you can customize IBM Connections by changing the user interface, adding features to applications such as the home page and profiles, integrating profiles with external data, and exploiting the IBM Connections API, among other aspects.
For questions and answers I used the IBM developerWorks forum. As of now there seems to be more traffic in this forum than on StackOverflow.
The community decided to use #ibmcnxdev as tag for technical IBM Connections related tweets so that you can follow Connections news without having to follow all potential people individually.
In addition to these key resources there are some more technical and non technical sites and services. For example Stuart McIntyre mentions in his blog a Connections LinkedIn group and the Connections users community.
For developers to get started I suggest to use one of the available development images: Quick start image on IBM SmartCloud or images for IBM business partners. If you only want to try the REST APIs remotely you can use the Connections instance on Greenhouse.
Furthermore Paul Mooney and Gab Davis describe on connections101.net how to set up IBM Connections.
There is a also a page on IBM developerWorks giving an overview of the IBM Collaboration Solutions Technical Enablement.