01 See also #TiSDD 3.4, System maps, and #TiSDD chapter 10, Facilitating workshops.
02 See #TiSDD 6.7.3, Ideas from future-state system mapping, for more on stakeholder constellations and Investigative rehearsal in 7.2, for an explanation of investigative rehearsal.
Define a specific perspective (e.g., from a customer’s or an employee’s perspective) for each workshop and invite participants with a sound knowledge of the ecosystem either from a shared perspective (such as customers for a customer’s perspective) or from differing perspectives (such as various internal departments if you want to map the internal stakeholder system). It helps to have a clear scope (e.g., a specific situation within a journey map) as well as the situational context if applicable (e.g., weekdays during daytime). This will help workshop participants to get on the same page. 
In addition to the know-how of the workshop participants, a second important factor for any co-creative workshop is the qualitative research you do beforehand. As a rule of thumb, the more valuable data you bring to a co-creative workshop (through a research wall, a simple mind map, or a research report), the more representative your outcome will be.
Be careful if you conduct workshops like this with participants who only have a superficial or abstract knowledge about the system. The results might look convincing, but often they are very biased. For example, if a management team without prior qualitative research and without deep knowledge of the daily lives of employees conduct a co-creative workshop on their internal stakeholder system, the outcomes tend to represent more their idealized organizational structure than the existing formal and informal network.