To claim an area open your inventory and click the map icon

Then just click the chunk on the map

to chunkload the chunk SHIFT+LEFTCLICK the claimed chunk

To give your friends access to your claimed chunks invite them with this command
/ftb team invite <username>

After you invited your friend, he/she has to run
/ftb team join <yourname>

If your friend is already in a team, you can leave it with
/ftb team leave