The "Turn Back the Pendelum" Arc is answering quite a few questions we thought up since the club began.
The most recent publications of the manga show how the current Vizard (with the exception of Ichigo) became what they are.
Spoiler: The "mission" starts out as recon to meet up with a separate Soul Reaper squad sent to investigate something, I can't remember right off the top of my head what it was. Anywho, the second group starts to become Hollows for some unknown reason. Shinji reaches them and he too becomes a Hollow. Then guess who shows up? Aizen, young Gin, and Tousen. Then Urahara appears and the guy who assist him at the shop. The guy uses a high level Kido on them but Aizen uses a forbidden Kido to block it.
I doubt Aizen actually "befriends" any Hollow to use them for his experiments to create Arrancar. Some might have common ideals as he does and choose to align themselves with him. Some might just have been taken due to his immense Spiritual Pressure. Others may have just chosen to become a Arrancar to become stronger.
Also, knowing Kiskue, I'm sure he counted on using Ichigo to have him become a Vizard so he could probably either redeem himself when he used Rukia to harbor the Hogyouku or just be strong enought to take on Aizen.
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