How to add records to related entity in workflow

Dear reader,

ever had that feeling that your quest is a common one, but you just don't see where to find the solution?

We have entity projects. Related (n:n) to projects are systemusers.
Whenever a new systemuser is created, I need to be sure that it's added to 4 or 5 specific projects. Since this is very important, we can't rely on manual action. So there I was, looking for a workflow solution.
Without any doubt this question has been answered a thousand times, but to google without the right keywords is like searching in the dark with a magnifying glass.

Any ideas?

thanks in advance
(sorry in advance ;) )

Jeroen