Yikes. Yeah you’re right, having ambiguous button text is not going to “prime behaviour”, it’s going to cause confusion.
A/B testing is one way to settle the dispute, as Ash and Natalie have suggested. Some people are only swayed by quantitative information.
What that test won’t tell you is why one label resonates with people compared to another. For that you need to talk to your users, observe them using the thing, and try and understand what their expectations are at that point in time. This might mean finding 6-8 people who legitimately fall into your target user group, and showing them two versions of the page (perhaps randomising which version gets shown first) and asking them what they think the site is about, what they think they can do there, and what they expect to happen if they click the button. And [I]would[/I] they click the button (users don’t always do what they say they do, so take it with a grain of salt).
What you should be trying to do is understand is where they’re at when they arrive at the page, and what these two labels will mean to them at that point in time. In general, the clearer the better—if you consider the user to be drunk, then “Add discussion” makes a lot more sense than “Engage”.