Party LineDownload Party Line
Phones
No app store needed: when your host has set up phones, the invite has a link for phones that opens Party Line in the phone's browser. Add it to your home screen to keep it.
Mac
Not ready yet. Tell us if you'd use it.
"Windows protected your PC"
Windows shows this for programs it hasn't seen many times yet. Click More info, then Run anyway. Party Line updates itself after that, and every update is checked against Party Line's own signature before it installs.
Check the file yourself
The SHA-256 of Party Line 1.2.5 (Partyline-1.2.5.exe):
c6665d014e3f7ad01051b5c83452f8677e3a751866efb802989505da6892d8d8
In PowerShell: Get-FileHash .\Partyline-1.2.5.exe
Got an invite?
Install Party Line and open it once, then click the link in the invite. If the link does nothing, copy the whole message, open Party Line, click Join with an invite and paste it there.