I think communication and shared pace-of-travel mode are important. It's also great when each party has a "special travel skill".
I travel best with my adult daughter. My skill are the details- I research extensively and plan the itinerary [I'm retired, so have the time] and she [very busy professional] regularly reviews the travel itinerary drafts and looks for issues I've neglected to research.
Then we're off-I've done the details, and she basically implements the itinerary.
My daughter's skills- she has an incredible sense of direction and travel calm. I have NO sense of direction; I can't even find my hotel room when I get off the elevator. I couldn't find our hotel in Rothenburg, and we were within one block of the main plaza. My daughter seems to just "know" where the right bus/tram/train track is, and stays calm when she's confused. She got us out of France December 2019 during the French transportation strikes, which caught both of us by surprise.
We also both have the same sleep and eating schedules: the last ones down for breakfast in the morning, but also the last ones back to the hotel in the evenings.
My husband, on the other hand, is up at 5:30am, frustrated that he can't get coffee, first in the breakfast room at 7:00am, obviously wants to be out of the hotel exploring asap, then ready for a nap just as I'm getting energized for the day. He has a lot of anxiety on travel days, those which require meeting trains and connections. And- he wants to be in bed by 8-9:00pm.
Somehow, we make it work, and continue to travel together, but I know which trips are best for the hubby, [less complicated itineraries, fewer transitions, outdoor activities, repeat countries] and which are best taken with the daughter [new countries, tighter agendas, museums/churches, some unknowns.]
Hopefully I've demonstrated examples above of individuals' travel differences, which need to be discussed for successful group travel.
Most important of all- I wish us all safe and healthy travel opportunities in the near future!