Application to display Location of mates on a map (image)

I am looking for someone who can write an app, which gets the current GPS position and sends it to a server (which is developed by me - so we just need to agree on the API, this shouldn't be a problem). It needs to display a map (image it gets from the server on startup) and shows own position and gets the position of others every few seconds (configurable) and displays them, too. The tool is intended to be used on a small area (like 1 square-kilometer) with about 300 friends. It shall show you where your friensd are and them where you are. I need to have the source and all rights with it, because I may want to open source it later or something. If you want, you can make that a condition, too - that it becomes an open source project. The freelancer doing this must speak english or german.