Italy and Slovenia logistics

Van or minivan or two cars will be needed. There is very little possibility for rail travel in Slovenia and Croatia. In north Italy: Dolomites and lakes, in Slovenia Ljubljana and Lake Bled, in Croatia at least Split, Dubrovnik, Plitvice National Park.

Two cars are necessary. However be aware that many rental car companies put restrictions on taking rental cars to Eastern Europe. Slovenia might be ok, but not so sure about Croatia. You need to check.

If you don't want two cars (likely large ones to accommodate that many people plus luggage), you could always use a bus, but I don't understand the question. Where are you planning to fly in/out of, and what is it that you wanted to see and do on this trip? I doubt you will have time to include Croatia, but that depends on what you wanted to see in Italy and Slovenia.

You definitely need to define yourself what you want to see in Northern Italy. People spend more than 12 days there alone so you will need to state exactly what you want to include for someone to help. Venice? Anything else; Verona, Lake Garda, Dolomites, etc...
Similarly Croatia is a big country, given your timeframe unless flying I don't think you can combine Northern Italy, Slovenia and Dubrovnik for example so Croatia could depend on where, if you want to include 2 nights in Plitvice Lakes area in Croatia, that you could probably make work but including Croatia at all could change a relaxing trip to a hectic one.
Slovenia is smaller so the places you likely would want to see are not that far from each other.

One important thing to try and figure out, assuming you are renting a car or 2 cars given the size of your party ; you will need/want to pickup and return in the same country ; doesn't have to be the same city but the same country for the return.

If there's 7 adults, opt for a VW Transporter-sized van, it will be the most comfortable option. That or two separate cars. Rail travel goes from good in Italy to passable in Slovenia to not recommended or indeed no railways at all in Croatia, depending on the destination. Crossing borders by public transport is probably the most annoying part as the cross-border connections can be horrible. Renting a van or two cars will be the better experience.

With just 10 to 12 days, visiting all three regions is not realistic. Furthermore, Northern Italy is a vast region that encompasses everything between the French and Slovenian borders and can easily accommodate three or four 10-12 day trips on its own. Generally, if you want to make it out of Italy at all in your timeframe, I wouldn't venture further west than Lake Garda. If you're not intent on rushing from destination to destination, I'd put the limit even further east, to Venice. If Croatia isn't a priority, I would drop it altogether and focus just on Italy and Slovenia. Alternatively, leave out most of Italy and focus on Slovenia and Croatia or opt for a single-country trip if you want to make it past the highlights and get off the beaten path.

It all really depends on your interests and travel pace, but to give you an idea for the Slovene leg, you would need at least four days to see Slovenia's highlights (ie. its most touristy places - Bled, Piran, Ljubljana and one of the caves) and if you want to make it to a couple of hidden gems like the Soca or Logar Valleys, budget another two to three days, and Slovenia is the smallest country on your list!

Annoying practicalities here...

1. You may need more/larger vehicles than you are getting advised to...you don't just need enough seats, you also need room for all their luggage! 7 people can squeeze into a large van. 7 people and all their luggage? Probably not. Likewise, 7 people in 2 medium size cars? OK. But not if everyone has two decent sized suitcases. Be honest with yourself about how much space/capacity you really need. Remember, you are not just squeezing in for a half-hour ride from the airport to your hotel...you're talking about all day in the vehicle...
2. If you rent a car in one country, and return it in another, you will surely get whacked with stunningly large fees. The fee for a small car is shockingly high. I don't know if the fees would be even higher for multiple large vehicles - you better check.

As everyone is saying, although Italy and Slovenia and Croatia are "right next to each other," they don't connect well on a single trip, without major advance planning and/or compromise. With only 10-12 days, pick just one or two countries, and make your life easier.

With such a very short time frame, I'd recommend skipping Croatia. You simply don't have time, and if you do make it there, it will have to be at the expense of one of the other locations. The main issue with Croatia is transportation. At that time of year, the budget airlines may not yet be operating seasonal service to Croatia. Also, rail service is limited in Croatia, and doesn't extend further south than Split, so getting about there requires lots of planning.

The most efficient plan would be to use open jaw flights, starting in Slovenia and returning from Italy (or in the reverse direction). Many of us here have listed our home location in the forum profile, which makes it easier to provide more specific details. As you haven't done that, it's difficult to get any idea of what your flight options might be to either location.

Using well planned public transportation would be an easier option than having to deal with multiple vehicles, drop-off fees or restrictions on driving in other countries. Keep in mind also that each driver listed on the rental form will require the compulsory International Driver's Permit for driving in Italy (and possibly in Slovenia too).

It's difficult to provide any suggestions without having some idea on which places your group is most interested in seeing. "Northern Italy" is a large area with many possibilities. What do you most want to see in Slovenia? You might find it helpful to have a look at guidebooks for both Italy and Slovenia, as that may provide some ideas.

Good luck with your planning!