The spice girls weren't the only or the first, but one of the most clearly and well-executed "music group" concoctions (cynically) aimed at a specific demographic.
Oh, they weren't remotely the first. A really old example is the Comedian Harmonists, formed in 1928. It was assembled after a newspaper ad calling for auditions, and the plan was explicitly to create a German equivalent of a foreign group that had impressed the originator, namely The Revellers. That was exactly how Boyzone was formed too, only there the foreign group was Take That.
Village People was also formed after a newspaper ad and auditions. There, the purpose was maybe not so much to make a localized version, as a subculture-inspired twist on an established concept.
That's how Spice Girls were formed too, as an obvious twist on the well-established concept of a boy band. But they didn't succeed in the market they were formed to target, which was teenage boys! Luckily for them, they were surprised by the emerging market of pre-teenage girls.
All these bands were formed after ads and auditions, but some of them - perhaps the most deserving of being called "boy bands" - were formed by managers or other people who weren't themselves performers.