Sensible group Id for individual project?

Take a look at the official guide:

groupId will identify your project uniquely across all projects, so we need to enforce a naming schema. It has to follow the package name rules, what means that has to be at least as a domain name you control, and you can create as many subgroups as you want. Look at More information about package names.

eg. org.apache.maven, org.apache.commons

A good way to determine the granularity of the groupId is to use the project structure. That is, if the current project is a multiple module project, it should append a new identifier to the parent's groupId.

eg. org.apache.maven, org.apache.maven.plugins, org.apache.maven.reporting

So, just as you would do for a package name, you should ideally use domain name, that represents the project. If you don't have one, you can just imaging your project having one. So "domain you control" might be your personal domain, or this specific project's domain. I.e. com.andyw.nextbigthing or com.nextbigthing. If you're considering it as something, that can be interesting to be picked up by community, you can skip the personal name and use org instead. I.e. org.nextbigthing.