Shimon Nodel,
“No, he’s supposed to go outside and look for three more people to complete the minyan, talking freely while doing so, then come back and say yishtabach and immediately start barchu without waiting for his three captives to catch up”
1. It is permissible to interrupt pesukei d’zimra to prevent a loss, to respond to a greeting, etc. I think attempting to secure a minyan for shacharis would fall into this category of permissible interruptions.
2. Captives? That’s quite the negative outlook.
3. What about tircha d’tzibbur? I don’t think it would be permissible to wait.