Write to hello@getwhenabouts.com · every message is read by the person who built the app. Including your macOS version helps; the app's "Send feedback" link fills it in for you.
Whenabouts lives in the menu bar: look for the two-circles mark near the clock. There is no Dock icon. Click the mark for the popover; settings and Quit are behind the gear, bottom right.
Calendar and Contacts access are optional. Calendar paints your own events into your band and powers "you are both free"; Contacts brings birthdays and photos. Deny either and the rest of the app works untouched. Change your mind in System Settings · Privacy & Security.
Whenabouts is free and full-featured for two people. Adding a third asks once for a $9.99 one-time purchase · no subscription, Family Sharing included. On a new Mac, the purchase follows your App Store account: the sheet's "Restore purchase" button brings it back.
People and settings sync through your private iCloud. Both Macs need to be signed into the same iCloud account with iCloud Drive enabled.