I have to disagree. If it's a social event and the host or hostess has made no indication that guests are not allowed, it's reasonable for people to ask about it.
The rules of etiquette have long shown us the ways to make it clear who is invited. And that's in the invitation. The person named on the invitation (and in the case of a wedding with a double envelope, check the inner envelope for more names just in case) is the one invited. Period.
ETA: Some invites do make mention of who can and cannot attend. For example, it's not uncommon to see wedding reception invites make note of age limits (saying something like "adult only reception"). You can always include a note that says "Unfortunately we are limited with room and are unable to accommodate additional guests." I, personally, wouldn't find that wording offensive and would solve the problem of people calling to ask. However, you still run into a potential mess if people realize that some spouses were invited and others were not.
Notations like "adults only" are for those people who don't know how it should be done. Who haven't read their Emily, or nowadays her granddaughter (or daughter?) in law Peggy, Post.
It's the same compensation that's been done by putting a self addressed stamped envelope with which to rsvp (and the rsvp form, for that matter)...people used to know, and have the stationery with which to do so, that when you get an invitation, you grab your pen, paper, envelope, and *your own stamps*, and you rsvp right away. Nowadays people have forgotten this, so now it's "tradition" to have the stuff included already.
Etiquette has rules for this...people just aren't learning it anymore...
OP, I wonder if you could really run your numbers there.
I know it would cost you extra. But the hall holds 120. You expect 100.
One imagines that your friend and family invitees already include spouses, so no wild cards will be coming from there.
So then you have 45 employees. Some of them are spouses of each other.
So subtract those couples already taken care of from the 45. Some surely are single and hopefully won't decide to bring people. So how many married or seriously commited couples are left? How many of them have already contacted you? How many do you think will really bring someone.
I wonder if you might end up only hitting that 120, even if you invited the spouses....
I know you don't want to spend that extra, but it might make people really happy.