Well, it's a little more complicated than that unfortunately.
There are like 10 different plugins I know to extend user fields, and each of them use a different way to store the extra data related to the profiles. While it seems fairly easy to just grab those data and index them, on the other side it's extremely complicated due to the nature of how WordPress stores users and user data.
On top of that the current state of user search with Ajax Search Pro is not the best, but luckily I have already implemented a much better one to the upcoming version. I still cannot promise though that it will be possible to search those fields.
The new will support the so called “user meta”, which is a database table for storing any amounts of key->value pairs for each user in the WordPress system. So if the membership plugin you are using uses this standard way of storing these data, then it will definitely work.Best,
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