In our Salesforce org, under "Activity Settings", the feature "Allow Users to Relate Multiple Contacts to Tasks and Events" is enabled. However, this seems to work only standard page layout when I click on "Log an Activity". We have a custom button to create an activity for a specific record type. On the name field (WhoId) on the page layout for the activity, it is presenting only a lookup popup where I can select only one contact or lead, However, on the standard page layout on actiivty, it is displaying a popup where I can select multiple contacts or leads (up to 50). Is there anyway I can show the same popup on the custom visual force page where I can associate the activity to multiple contacts or leads? Below are the screenshots of the custom page layout page popup and the standard page layout popup.

Lookup Popup on custom page:
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Lookup Popup on standard page:
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Can someone help?