Writing Gherkin scenarios collaboratively can be beneficial both for business analysis, discovering the language of the domain and for the purpose of driving the code.

For both use-cases, here are some of my rules-of-thumb for writing Gherkin scenarios effectively.
#bdd #ddd Given, When and Then are analogous to the three standard stages of a test:

Arrange: arrange the system to be tested in a given state
Act: take the action that is to be tested (perform the test itself)
Assert: check that what should have happened indeed happened