Multi Team Retrospectives can be very different than individual team retrospectives. Simply put, encompassing more people introduces challenges.
Various Agile methodologies that focus on scaling deem it necessary to conduct multi team retrospectives. For example, SAFe prescribes that an I&A (Inspect & Adapt) workshop is run at the end of every PI (Program Increment) which can work out to approximately every 3 months.
It can be a daunting task to hear the voice of over 100 people, facilitate conversation, and agree on tangible outcomes. If you are fortunate enough to be tasked with this, here are 3 suggestions:
#1 - Value Stream Mapping
Have every team value stream map their current process. You'll have to specify the boundaries of that process in the interest of time. Then have the teams value stream map their desired process. Have them contrast the two and come up with takeaways that get them closer to the desired state.
#2 - OpenSpace
OpenSpace is also referred to as an UnConference. The idea is that the event is agenda-less and the attendees decide on the agenda. Coming up with the agenda can be time consuming so you may have to poll the attendees prior to the event to come up with topics. Choose a few topics and run simultaneous sessions where each is conducted by a designated facilitator (e.g. Scrum Master). Attendees are free to use the "law of 2 feet" to move around to each session as they choose. Have each facilitator present the ideas that were discussed and as a large group decide on the SMART goals.
#3 - World Cafe
World Cafe has some similarities to the OpenSpace concept. Essentially, sessions are run in parallel however they are shorter in nature. They are shorter to allow the participants to move to the next session. This allows each participant to participate in each session with the same people. A designated table host remains at the end of each session to explain to the next group what was discussed. The table host is not the facilitator. There is no facilitator. You could say the group is the facilitator. World Cafe forces the group to talk to one another.