It depends, I have been in bands where I considered them coworkes, other bands where we were all friendly but not necessarily friends, and in my current band, absolutely we're friends. Again, really just depends on the situation.
As far as your situation, I would just look to attend the party since you were invited, look at it as keeping the peace / band politics / not upsetting the apple cart. Show up, bring something, stay for an hour, maybe a little more. Let your band member know when your leaving and that you appreciate the invite, then politely excuse yourself, work, or some other obligations. I mean after all, some work places have social gatherings that you might attend for the same reason, just look at it that way.