If you try to install the package and get the following error: This app can't be installed. There are problems that prevent this package from being installed. Missing Organization Feature: ManyWho
Don't worry. It just means that you need to go back and do the pre-installation step for enabling "Allow Users to Relate Multiple Contacts to Tasks and Events" in the "Activity Settings" section of your Salesforce Setup panel.
This process is quick and easy and is shown step by step in another article called "Before you download Appointiv from the AppExchange"
Search Tags: Many Who, AppExchange installation