Workflow (or whatever) to associate n:n entities

Dear reader,
I'm struggling with a new requirement looking like this:

We have a n:n relationship with custom entity Project and systemuser (used as project teams).

As soon as a new CRM user is added, he/she should be added to several (10+, so a lot of manual labor with all possibilities for errors) project teams.

I would like to refrain from database triggers and SQL inserts (possible, since we have all on premise) and my client won't pay for a plugin developer (can't do that myself unfortunately, unless someone could point out a clear HowToForDummies).

Is this even possible in a workflow or using javascript? Any other ideas?

Thanks for thinking along with me!
Best regards,
Jeroen