I will look into making it possible to sort by join date as an alternative to the default username. It is not a trivial request due to the alpha paging and numeric paging and the number of database platforms we support so I'm not going to make it sortable on every possible thing but a way to easily find new members by sorting seems reasonable.
The search works differently for admin users vs other users, for admins it does search by username or email. I can potentially expand that to also allow admin searches to include first and last name though those fields are not included in the user profile by default currently so in most sites including this one they are blank fields. I may change the default to include those fields on the registration/profile but not require them. Site owners can make it required by using a custom profile config file.
Allowing any site member to search on email, first and last name would be leaking secure data by confirming that a user has a specific email address, name etc. Might be ok for an intranet but not for a public site. I will consider making it configurable to allow that for intranet scenarios and an option to include those fields in the member list also for intranet scenarios.
I'm not making any promises about when I will get to this, but I will log these things in the project tracker so we will get to them.
Best,
Joe