Volibear can be the first or last to enter a team fight, with this specific build he would be better to go in first as a tank but would still do damage as a support if he is ignored by champions on the other team. If you get into a team fight open with rolling thunder and target the most squishy champion, then use majestic roar to keep them from running away at normal speed, then use thunder claws to deal damage to the other champions and as soon as you get your third hit use frenzy. Frenzy will deal the most damage out of all your skills if you follow the skill chart.

3: resistance
3: hardiness
4: durability
3: vigor
1: veterans scars
2: Indomitable
3: evasion
3: Honor guard
1: juggernaut
1: summoners wrath
3: brute force
1: butcher
3: alacrity

The item selection is mostly made for a tank who can still deal fair damage, 2 or 3 warmongs will give you a huge amount of health and the thornmail will allow you to deal passive damage to all champions attacking you equal to 30% of their damage. You can easily substitute one or two of the warmangs for more magic resistance or armor. But I suggest at least two

I use teleport and flash, Flash should be used because it allows you to escape a battle if needed, and the tele spell I use just to move around the map quicker but it can be substituted if you feel it is unneeded as well as flash but I suggest flash and another spell of your choice.