Don't assume that their assumptions are set in stone or even that they have a set of assumptions about the activity. Groups you see as adversaries may be potential partners. Find something that potential participants can all agree on. This will focus the study, give people something concrete to work on, think about, and get excited about solving. Worth the risk .
People risk personal or organizational credibility if the process fails.
Understand these risks, paint a realistic picture, and explain why you think it is still worth their risk. Bring them out to see the problem first hand. Working a site or officevisit into their travel schedule provides a concrete idea of what you are trying to do. Find their values and motivate through talking informally and looking at their other activities. Either demonstrate how your solution fits within their agenda or look elsewhere for commitments.