What's your planning process for big group trips?
My university friends and I (and everyone's significant others, so about ~15 people) are planning a big catch-up trip, which will also be the first time to the country where I live for all of them. Planning has been a little higher friction than I expected, because we want to coordinate travel times to specific cities ("let's spend 4 days in City A, then all go to City B" etc), but people also have specific activities only they want to do (scuba, theme parks, etc) within each city. However, there's way too much to choose from and there's no way we'll do everything that everyone wants to do.
So right now to gauge interest in specific cities and attractions within them, we are just using a private Facebook group where people just make an idea as a post and people vote by liking it, and people can discuss the place in the comments. Things that we have fully decided are just placed on a spreadsheet. This process works but it doesn't feel great.
I've also explored Wanderlog, and I really like the fact that you can easily search for stuff and then place it on a map, but unfortunately its UI doesn't really support "branching" trips where some people will do different things on a given day.
What tools/processes have you had success with when trying to coordinate a big group trip?
Contact potential places you'd like to stay (hotels, camping sites, whatever applicable for the different areas) and ask for a quote for X amounts of people staying Y amounts of nights for the dates you'll be there. They'll likely have rates for both single occupancy, double occupancy, and double rooms with extra beds/cots/sofabeds set up.
If you get rates you like, you can book with them with a cutoff date. The place will give you a booking number that is locked to the prices you were given for the different room types - until the cutoff date.
Then let people contact the places directly using the booking reference number and their own credit cards for guarantees. That way, people can manage their own stays directly with the places you'll be sleeping rather than have 1 person try to act as middleman for everyone.
The reason for having a cutoff date is that once you reach that date, unused rooms will automatically be freed up for regular sale without you being responsible for remembering to contact every place to cancel unneeded rooms. If you forget to cancel regular bookings in time, you'll be charged for the rooms regardless of actually using them.
A group of 15 pax will usually mean you'll be considered a group booking and get you better rates if you book directly through the individual places.
When you have multiple places you'll stay, you might even get a better price for several of the places on your trip because of chain policies. Some chains around the world even cooperate despite having different customer facing branding, due to being owned by the same mother company.
Having your sleeping arrangements in place means you will be able to meat up every morning and afternoon/evening despite doing various activities during the daytime.
Lots of hotels/other accomodations also cooperate with local venues and might give you reduced prices for tickets.
Same applies for other places. Contact them directly and ask for a quote for groups up to 15 people for the dates you'll be there. They'll get back to you and you can then choose where you'll go.
I suggest a very basic notebook approach
"Headline: City we'll stay, place we'll stay booking no: XXXXYYYY"
And then a subset of headlines with day, options for activities and pricing, letting people write themselves up under what they want to do. If pre-buying tickets is available, then same process as with hotels: let people contact the venues directly to pre-pay, else just pay on the day.
People tend to do best with remembering their own plans though, so as the manager of a bigger event like this it makes it easier for you to provide the framework, then letting people manage their own minute details.
Caveat: I coordinated group activities professionally for a while both in the hotel and the travel industry, and 15-20 pax isn't that big of a group in my mind. I do remember it seeming so overwhelming in the beginning though, so I understand if it seems like it needs to be managed in minute detail.
In addition to the excellent advice u/DrEvergreen gives, instead of trying to plan every single day for the group, I would recommend planning days at location, with "we'll meet up at XX for brunch, then YY for dinner at [times], so that individuals/small groups have increased flexibility to plan their own one off adventures, but still remain part of the larger group for key times.