Yes, Howbout's shared calendar is completely free to use on iPhone and Android.

Howbout is a shared calendar made for friends. You can add plans, instantly see when everyone’s free, color code plans and friends to keep everyone organized in one place. You can also sync in other calendars, like your Google or Apple calendars, so that your work events or subscribed calendars automatically appear in Howbout. That way you can track your full schedule alongside your friends’ plans, making it easier to find when everyone’s free.