A group begins with clear agreements: confidentiality (what is said in the group stays there), respect, regular attendance and everyone's right to speak when they are ready. The therapist holds a safe space and keeps the conversation useful.
Some groups are structured, with a skill or topic each session, such as CBT or DBT skills groups. Others are more open and work with what happens between members in the moment, because the way we relate in a group often mirrors how we relate in life.
Therapists usually talk with each person before they join to make sure the group is a good fit.