WhatsApp two groups members intersect

diff.js

(() => {

  const B = '+7 901 555-55-55, +7 903 777-77-77'
    .split(', ').sort();
  
  const A = '+7 901 555-55-55, +7 926 222-22-22'
    .split(', ').sort();
  
  const both = [], onlyA = [], onlyB = [], lA = A.length, lB = B.length;
    
  let iA = 0, iB = 0;
    
  while(true) {
    
    if (iA >= lA) {
      onlyB.push(...B.splice(iB));
      break;
    }
    
    if (iB >= lB) {
      onlyA.push(...A.splice(iA));
      break;
    }
    
    const cA = A[iA];
    const cB = B[iB];
    
    if (cA > cB) {
      onlyB.push(cB);
      iB++
      continue;
    } else if (cA < cB) {
      onlyA.push(cA);
      iA++;
      continue;
    } else {
      both.push(cA);
      iA++;
      iB++;
      continue;
    }
  }
    
  console.log("%d both: %s", both.length, JSON.stringify(both));
  console.log("%d in A only: %s", onlyA.length, JSON.stringify(onlyA));
  console.log("%d in B only: %s", onlyB.length, JSON.stringify(onlyB));

})()

To grab a list of group members from WhatsApp web, open browser's Development Tools and "Inspect element" that displays group members on top of the dialog window. Copy Inner HTML of that span - it has phone numbers or names of the users from your phonebook.