The Glympse app allows you not only to share your location, but also request the location of someone else.
To request someone's location, follow these steps:
3. Select from your contacts or enter the phone number of the person who's location you are requesting. You also have the option to add a destination if you choose.
4. Click send within the Glympse app. When your iMessage screen comes up, send the text (SMS) message with the request. The recipient will then receive the request, and can choose to share with you, or to ignore the request.