It’s good that you are using group work and you are right to say that it can sometimes be difficult to get students moving. I’ll first consider interesting ways to group students and then look at how to help the activity go more smoothly.
There are a variety of ways of putting students into groups; you could consider any of the following:
All students who have their birthday in January and February work together. Those who have their birthday in March and April work together, etc.
All students wearing black shoes work together, those wearing brown shoes work together, etc.
People who come to class by bus work together, those who walk to class work together, etc.
Students with brown eyes, green eyes work together.
People who prefer skiing holidays work together. Those who prefer beach holidays work together.
There are many other options like these that you can invent yourself. You can even link it into the topic of the lesson. For example, in a lesson about the family, you could group people from families of three children together, those from families of four children together, etc.
These are all novel ways of grouping students but you will need to organise it carefully. If we take the example of eye colour, first of all ask those with brown eyes to put their hands up. Tell them they are the brown-eyed group. Then do the same with green eyes and blue eyes. You then designate areas of the classroom as being for brown, green and blue-eyed people. Check everybody understands where they should be going, for example by asking 2 or 3 students what colour eyes they have and then asking the group where those students should go. Tell students that when you give the signal, they are to move to that area of the room and not before. Make sure you leave enough time to give clear instructions, to check understanding and leave plenty of time for students to actually move. You might also want to tell them what to take, e.g. just a pen and paper. Some students might start moving everything, including coats and bags.
Another thing to consider is that in any of the examples above, you will not get groups of even numbers. Some groups will be bigger than others. You might need to ask some people to move to another group, but that breaks up the identity of the group. You could also split a large group into two smaller groups. For example, two groups of brown-eyed students, one group of blue-eyed students, etc. Also consider your choice of grouping carefully. For example, in some cultures, a large majority of people have brown eyes; so this type of categorization wouldn’t work. Splitting people up according to form of transport used wouldn’t work in some towns.
If your students are resisting group work, you might want to have a few lessons where they simply work with the people sitting next to them. This will get them used to group work and also save time.
I hope you and your students will enjoy these activities.