Our party had a fight last night that consisted of:
6 x bugbears with each one having over 40 hit points
2 x Bugbear sorcerers\clerics
2 x Giant bats
1 x Bonedrinker that has spell resistence and spells ??
Whats the challenge rating of this encounter?
Luckily we managed to split the encounter with 2 walls of fire
Our average party level is 6 !!
Without knowing the levels I would guess somewhere around CR8, mostly due to the Bonedrinker. 2 Giant bats at CR2 are CR4, and combined with the bugbears might work out to a CR 6 encounter. If the Bonedrinker is CR6, the sum total would work out to CR 8. This is all estimation, of course, but I think even worst case with a few spellcaster levels on the Bonedrinker the CR would be know higher as 10. As FallofCamelot said, your 6th level party should be able to take on CR10, especially with proper tactics like splitting the encounter.
Good job, though -- you fought a superior enemy and came out ahead. Way to be heroic!
If the Bugbears had over 40 hit points each, then they were likely around 3rd-level (assuming average hp rolls and one +4 adjustment placed in CON). So each bugbear would be at least CR 5 if they had PC-classes (possibly CR 4 if they were 2nd-level barbarians).
A total of 8 CR 4 bugbears would alone be a CR 10 encounter, so the full encounter would be around halfway to a CR 11 encounter (or a CR 11+ encounter if the bugbears were CR 5 each).