Friendz can respond to two different URLs:
friendz://add?gamertag
As the URL implies, this will tell Friendz to add "gamertag". There is no need to replace spaces with a plus sign.

friendz://message?gamertag
This URL is for sending a message to a gamer. To send to multiple gamers, separate gamertags with a comma (,)

Nothing to it! See examples below:
Add Thermogl on Xbox Live
Message Thermogl and Someone else on Xbox Live