I have a sexually-active relationship with a woman who I have been seeing about once a week for almost a year now. We have other partners and are pretty cool about our open relationship. Just this past Friday we had a
great night out and she stayed over my apartment. However, I had
out-of-town obligations I couldn't avoid and left her to use my place for
the rest of the weekend (she lives in NJ). Turns out she brought a male
friend/co-worker back to my place to stay Saturday and Sunday night. I
really don't know if anything went on (she says it didn't and I trust her to be honest about these things) but I am pretty sure they shared my bed. Am I
wrong to think of this as disrespectful?

Steaming in Staten Island

In a word, NO. This was disrespectful, both because she utilized your personal space to host the overnight stay of a friend, and, even more significantly, because she did not ask your permission to do so prior. No guest should make assumptions about their entitlement to a host's space without checking in prior. The fact that she brought a guy back to the apartment adds insult to injury; but from the advice-giving standpoint, we'd advise you to approach the issue by bringing up first the fact that she did not ask your permission to have a guest, and later you can introduce the guy into the conversation. This way you'll ensure that the whole "open relationship" conversation can remain its own topic (which, yes, indeed needs addressing) and you can still retain focus on the issue of "respect for your place." It's an issue of respect all around.

With regard to the open relationship, you say that you "trust her to be honest", but at the same time you don't sound certain about her behavior while you were away. You should probably use this oportunity to establish between you your own "rules" for the relationship; that is, do you require the other party to inform you about their other 'involvements'? Some people use a don't-ask-don't-tell rule, which may work but can be both physically unsafe (you want to ensure you're not being exposed to any STD's, etc.) and emotionally challenging (where are the lines between disinterest, curiosity, suspicion, and jealousy?). Others disclose full information about external encounters, and for many, some area between these extremes works best. The important part, though, is to be on the same page as your partner(s).