Clear add member on successful add

(cherry picked from commit 3276a50c)

pasapp/static/pasapp/js/group_edit.js

@@ -34,6 +34,7 @@ function addUser() {
             } else {
                 userBox.innerHTML += `<div class="user-bubble" onclick="removeUser(this,'${username}')">${data.name}</div>`;
                 
+                input.value = "";
                 userSet.add(username);
                 errorBox.classList.add('hidden');
                 updateForm();