This question is the most important one but at the same time the hardest to answer.

If all members of your group have the same opinion about a destination, it should not be a problem. The same is for a group in which the leader solely decides and makes his/her proposal which will be accepted.

However, as soon as there are different opinions and interests in your group things have to be laid out, destinations researched and topics discussed. You should gather opinions and interests first and set up a list of the 5 most popular destinations within your group. You can set this list up according to contintents, countries, areas or even according to Scout Centres, if your group likes to stay in an activity centre. You might even have a foreign fellow Scout or Guide group which you would like to visit, which will give your tour a focus. You then have to discuss the cons and pros, regarding the lenght of the flight/transfer in relation to the duration of the stay, the budget (for that some rough research into flight and accommodation costs are very useful), health and safety aspects, etc.