You can help BOLTS by just trying it and giving feedback. Whether you are using FreeCAD or OpenSCAD, just install and use it and tell us what didn't work as expected, what you think could be done better or how BOLTS could be made more useful for you.
You can reach us by writing an email, posting in the BOLTS development thread on the FreeCAD forums or use the BOLTS issue tracker on GitHub.
You can also help with adding more parts to BOLTS or improving those that are already there. To do so, you should first read about a few basic things, that will be useful to know
After that you are well prepared to contribute parts, data or drawings. For details on how to do that see the contributor documentation.
BOLTS scans its database for inconsistencies, common problems, missing or unclear information and automatically generates a list of things that could be fixed or improved. This is a great starting point for contributing to BOLTS.
If you notice errors, translation problems or other problems on the website, you can directly report them using the comment field embedded in many pages.
Also don't forget to tell all your friends, colleagues and fellow makers about BOLTS. Or blog about it, mention it on social networks and spread the word.