Windows runas command with elevated privileges, user in admin group

It looks like this utility - Hidden Start - will do what you wish if you use the /uac switch. Here's an example command line:

hstart /UAC "notepad.exe"

This will pop up the UAC dialog rather than ask for a password like runas does.

sudo always requires the executing user's password (and requires that you have specific permissions to do this, i.e. are one of the sudoers).

su requires the password of the target account (root by default, but root account has no password on OS X by default). If you use su instead, you can enter the destination account's password and execute a command using that user's privileges.

su -c some_cmd # as root
su username -c some_cmd # as username

^{This works by passing all arguments after the user name to the destination account's login shell. Shells usually support -c <commands> arguments. In GNU coreutils su, there's an actual -c command argument to su that can be placed before the user name.}

You can su to another user account (using the other user account's password), and sudo from there, provided that other account is a sudoer.

If you want neither to enter another account's password, nor give your regular account sudoers permissions, you're pretty much out of options unless you consider SSH with key authentication or something like that.