When some of the participants know each other from before or recognise that they have shared opinions, they may set up a subgroup of their own by always wanting to sit together in the workroom, always doing group work together, and spending all their free time together. Sometimes this closeness makes them avoid confrontation amongst each other or leads them to always take a joint stance supporting each other, which may disrupt the work of the training. You should not be too concerned over subgroups even when they are somewhat disruptive, but you should make sure that your methodology imposes group diversity, that you shuffle the seating arrangement, that small groups are selected randomly, etc. Bear in mind that the aim is not to prevent intensive communication within the subgroup, but simply to encourage a similar intensity of interaction with the rest of the group, or to transfer the mode of communication from their free time back into the workshop. It is natural that people within a group should feel different levels of closeness, and that in itself is not a problem. You should calmly resist the paranoia that sometimes occurs with less experienced team members who may see these situations as premeditated acts of subversion.