Uh, since you are in this week next monday would equal the 1st. So unless half your office is in some kid of time warp to next week already, next monday couldn't equal the 8th.
I voted the 8th. I'm not time warped. At least, I don't think I am.
I'm also kind of split and have always been confused by this too, but I've noticed a trend where it seems once you have passed a specific date (Monday) of any particular week, if the monday immediately in the future is to be cited for any reason, it would simply be like "on monday we'll be doing blah blah". A connection of future action plus simply stating the exact day of the week typically refers to such day that will be coming up.
Next typically means skipping one, though some serious confusion can happen when you are on a Tuesday of a week, and you say "next monday", because the very next monday is quite far away as it is (in general terms).
If it's actually Monday, and you say next Monday, it is almost universally understood to mean the upcoming Monday.
Saying "Next Monday" on a Friday, would lead me to believe they are not referring to the Monday immediately following the weekend.
But really, every single person has a different use of the fine details of the language, so any good leader would skip such vernacular and simply state "Monday the 1st" or "Monday the 8th".
Also when this is brought up in the first place, if anyone is even slightly unsure about the specific date, fucking ask the question when it is brought up. I cannot believe the office is simply voting instead of just asking the boss when the question came up.