Very often the 7th house is used as some kind of placeholder for people that you have difficulties assigning a specific house to, people that fall into the category of 'any old person'. Also, the 1st house is generally known as the house of self, and the 7th as the house of others. So if you've got a mixed group, then that certainly would be the 7th.
In reality it can get rather confusing because very often colleagues are also good friends and engage in activities together off work. So let's say you have a colleague/friend that works in another department, someone you also play tennis with after work and consider to be a good friend as well. If you ask when will she get the work done you forwarded to her, she will get the 7th because the context is work. If you ask if she will play tennis with you tonight as usual, then she can get the 11th, because the context is the off work relationship, which has already been determined as friendship. And if you ask her about a work related problem in an off work setting, then it can get really confusing, because she could choose to answer as a colleague or as a friend. Sometimes people even say things like "As a colleague I would suggest A ... but as a friend, just between you and me, I would suggest B." So you have to pay attention to such small details.
Context is very important in horary. Context will determine which house you have to choose. Very often, when it isn't clear, you can also check with receptions if you've got the right significator. And using a bit of common sense usually will do the trick as well. Going by the books, especially rulership books can be very confusing.