Users with Google's IP address. How is it possible?

What you're seeing is the Google proxy address.
Mobile users with a chrome browser (either Android or iOS) that have the bandwidth management features turned on will often be seen as using one of these addresses as the requester as described here.

In essence the data you're serving is being requested by the Google Data Compression Proxy, optimized and sent back to the end-user.

What they have to do to use their real IP address.

They shouldn't be doing anything differently.
You can check the x-forwarded-for header as explained in the previously linked documentation.

Probably they are using Google data compression proxy (https://developer.chrome.com/multidevice/data-compression).

And to answer your question (from the same page):

As a site owner, how do I perform IP geo-targeting?
The IP address of the mobile device is forwarded to the destination server via the X-Forwarded-For header. Site owners should check for this header to correctly determine the location of the user based on client's IP address.

You can get the user's IP address directly if you simply serve the site over HTTPS.

You probably should be doing this anyway - especially since you mentioned these are login and registration pages.

Quoting from the Data compression Proxy page mentioned in other answers:

Is my secure traffic optimized by the compression proxy?

No, data compression proxy operates on non-encrypted traffic: HTTPS requests are sent directly from the mobile device to the destination server.

Maybe these users use Chrome (mobile) with Data Compression Proxy ( https://developer.chrome.com/multidevice/data-compression )

You can use X-Forwarded-For HTTP header to geo-locate user based on originial user IP (See FAQ)