I created a mutation that adds a user to group, which call 'updateGroup' and that does trigger the subscription

const addUserToGroup = (root, { groupId, userId }, ctx, info) =>
  ctx.db.mutation.updateGroup(
    {
      where: { id: groupId },
      data: {
        users: { connect: { id: userId } }
      }
    },
    info
  );