1 Corinthians 5:9-13
9 I wrote to you in my letter not to associate with immoral men; 10 not at all meaning the immoral of this world, or the greedy and robbers, or idolaters, since then you would need to go out of the world. 11 But rather I wrote to you not to associate with any one who bears the name of brother if he is guilty of immorality or greed, or is an idolater, reviler, drunkard, or robber--not even to eat with such a one."
This seems to indicate that as long at the prochoicer is an unbeliever, we're in the clear working with them, but if they profess to be Christians, we're not to associate with them.
1 Corinthians 10:27
If one of the unbelievers invites you to dinner and you are disposed to go, eat whatever is set before you without raising any question on the ground of conscience.
This would certainly indicate that there's no prohibition against fraternizing with unbelievers -- even idolators! After all, if we refuse to associate with them, how are we to bring the Gospel to them?
I'm also looking, but can't find, the admonition to think the best of people. It's in one of the Pauline epistles, isn't it?