First you need a personal key. Create one via the menu in "Keys" or import existing secret keys. Afterwards, you can download your friends' keys or exchange them via QR Codes or NFC.
On Android lower 4.4, it is recommended that you install OI File Manager for enhanced file selection. To share via QR Codes install Barcode Scanner. Clicking on the links will open Google Play Store or F-Droid for installation.
Please report the bug using the issue tracker of OpenKeychain.
If you want to help us developing OpenKeychain by contributing code follow our small guide on Github.
Help translating OpenKeychain! Everybody can participate at OpenKeychain on Transifex.