Click on your FTB World, then click on the Configure
button.
On the configuration screen, you will see a Create Invite
button. Click on this to go to the world management screen.
You will now see 2 options, one to create a single use invite code and another to create a shared invite codes.
After selecting the type of invite you want to create, you will be taken to a screen that lists all of the current invite codes for your world.
From here you can click on the invite code to copy it to your clipboard and share it with your friend(s), or click on the red -
button to delete the invite code.
Joining a friend's FTB World is easy, all you need is the invite code they have shared with you.
Now click on the FTB Worlds button on the main-menu of the modpack, which will take you to the FTB Worlds screen.
Click on the Join World
button, then paste the invite code into the text box and click on the Join FTB World
button.