Adding a friend
Add your friend using Essential Mod.
In Main Menu on the right side click Social > Add friend > your friend's username > Add
Then your friend needs to accept invitation.
Hosting a world
Method 1 - To host a world, click 'Host' in the main menu, select world you want to host, set desired options, click next and invite your friend from the list, and click 'Host World', then your friend have to accept the invitation and will join your world.
Method 2 - Open your world, pause the game, click 'Invite', set desired options, click next and invite your friend from the list and click 'Done', then your friend have to accept the invitation and will join your world.