If you had to choose, Would you mix intervals (5x8 threshold) with a social (but big mtb climb) with friend? Or do intervals separately before knowing they will be quality, then go tackle climb w cooked legs? Any harm in climbing hard after doing threshold intervals?
Sometimes social and training clash!
I’d pick one or the other. And likely, I’d pick being social. I don’t know the MTBer etiquette where you live, but in the South of Germany, you climb at your own pace but wait at periodic intervals.
So you could still train, but again, I’d prioritize being social 
PS I don’t know about you, but if I think about it, I could always use more social rides. Especially during the pandemic, my team’s activities were mostly put on hold. With two small kids at home, I rarely get the opportunity to ride socially. Whatever small fitness benefit you’ll be missing, I am sure is more than made up for by an elevated mood, companionship and seeing the fruits of your training.
As mentioned above… be social. It’s good for the soul. Crush the climb and wait at the top. Have fun and don’t overthink it. 
Think I’d decide depending on the social group. If you’re among the stronger riders, train first and then take the climb really easy, maybe with the slowest riders. If most of your social group are stronger than you, I’d save my legs for the “social” ride and not train.
I wouldn’t try and do intervals during a social ride. You’re unlikely to be able to focus properly and do good quality intervals, and likely to draw a lot of stick from your mates!
Yeah, agree. I have a social group I ride trails with where the tempo is very low for me. So i either join on “rest” days/endurance days, or if logistics match up do some intervals 30-45 minutes ahead of time, and join the ride afterwards. Works for me.