adjust i18n

resource/locales/en_US/admin/admin.json

@@ -223,7 +223,7 @@
     "invite_users": "Invite new users",
     "click_twice_same_checkbox": "You should check at least one checkbox.",
     "invite_modal": {
-      "emails": "Emails",
+      "emails": "Emails (Possible to invite multiple people by emails with new lines)",
       "invite_thru_email": "Send invitation email",
       "valid_email": "Valid email address is required",
       "temporary_password": "The created user has a temporary password",

src/client/js/components/Admin/Users/UserInviteModal.jsx

@@ -49,7 +49,7 @@ class UserInviteModal extends React.Component {
         <label> {t('admin:user_management.invite_modal.emails')}</label>
         <textarea
           className="form-control"
-          placeholder="e.g. user@growi.org"
+          placeholder="e.g.&#13;&#10;user1@growi.org&#13;&#10;user2@growi.org"
           style={{ height: '200px' }}
           value={this.state.emailInputValue}
           onChange={this.handleInput}