I'm writing a web-app which has a form so that users can input the details of Thing A. Thing A is associated with at least 1 Thing B, which are also user-defined. Is there a pattern or some examples of forms which have this kind of relationship, where users can 'add another' Thing B to their Thing A in a form that grows organically to the users requirements.
The below images are a "flow" for how Google Compute Engine does something similar to what I think you are getting at. It has inputs for thing A and then has a Thing B section where you can fill out info for one, but also add a new one. When you add a new Thing B it collapses the first Thing B into a table like format showing a read only state of a couple the options the user filled out. It can then be re-expanded and edited.
If you were thinking of going a route where the user isn't creating a new thing B in the Thing A process, then, depending on the # of Thing Bs I would have an "attach" button that opens a modal with the list of Thing B's with checkboxes. The user selects the ones they want and adds them at which point modal closes and a summary/small table of the added Thing B's is shown.
Hope this helps and good Luck! (Please let me know if this is way off the mark haha)