There are various ways to get involved via GitHub : https://github.com/eclipse-thingweb
You can pick one of the Eclipse Thingweb components such as node-wot or playground and start using them. Even better, you can contribute to their source code! Below are some examples of what you can contribute with:
All Thingweb components are licensed with Eclipse Public License v. 2.0 and W3C Software Notice and Document License (2015-05-13). You can pick the one that suits your requirements (i.e. you only have to comply with one, not both).