Create an account in Kemble Circle now asks for a username. It is suggested from the name you show, never from your email address, and the form says whether it is free as you type. User settings › My account has Username with Edit: choose a new one, confirm it is you, and it is yours.
A message now keeps which person each mention was written for, so changing your username breaks no earlier mention of you, and a name somebody else takes later is not read back as them. An account made without a username gets one from its display name, not from its address or phone number.