Organizing a group trip may be the best way to accomplish some of your goals.
Where to go
Sometimes the destination is obvious, as your group is only interested in the old neighborhood or a particular event. In other cases, there's a general theme to get your started, and you need to narrow it down from "Let's all go to a beach somewhere".
Other times, the destination is unimportant, so long as everyone will be able to attend. In that case, consider factors such as travel time, time zones, and the total travel cost. For a widely scattered group, the obvious big-city destinations may also be the most expensive.
While many tour groups seem to naturally tend towards renting a bus (possibly including a driver) this has its own drawbacks and may not even always be the cheapest option. Buses are often rather limited in terms of speed and can be uncomfortable over long stretches. Deutsche Bahn is among the railway operators with offers for groups, ranging from Ländertickets for up to five people, offers from 19.90€ per person for groups of 6 or more people or an entire special excursion train. The Spanish RENFE has the "mesa" (table) offer where you buy a table and the four seats around it for a flat rate no matter whether one or four sit on those seats. In general ordering well in advance gives you the best offers and enables you to compare several offers. Compared to flying and driving, train travel has the advantage of letting people in the group walk around and talk to each other on the way.
Watch the money
If you're organizing a group trip for business, then the money problem is fairly straightforward: estimate your costs, get your budget approved, figure out which things the business will pay directly, and which things the individual employees will need to file as travel expenses afterwards. It's also simple when one well-off person is paying for everyone else, such as when parents volunteer to take their children and grandchildren on a trip.
Outside of those circumstances, handling money for a group is almost always more complicated. In a group of any size, you will have people who want to do everything as cheaply as possible, and others who will want to spend more lavishly.
- Ask people to tell you their budgets and preferences candidly.
- Send each potential participant a written estimate of the per-person costs. Specify things that are not included, such as transportation to or around the destination, certain meals, sightseeing excursions, or a private hotel room. Tell them when and how they will have to pay their share. Encourage people to drop out early, if your realistic estimate of the costs seems too high.
- If there are some shared costs that need to be paid in advance, such as a fee for reserving a banquet room, then collect that money before the trip begins.