Via email. Likely reproduction instructions:

• Create a bot account a@b.com on your instance.
• Invite a@b.com via the manager-facing "Invite Members" tool.

Actual behavior:

• Get an error about "contact staff to import", since the email is already associated with an account.
• This is correct-ish, but not very specific/helpful.

Desired behavior:

• Get an error about "you probably accidentally created this user as a bot, contact staff to delete the account".

See also T9304 and T9303 for root cause issues (it's currently very easy to ignore the warnings and create bot accounts when you don't mean to).