There was a similar question about a month ago - tracking employees with start and exit dates - and I recommended using self-join.
Take a look here: Using two date fields to create trend across time
There are additional links in the article which may be able to help.
Once you set up your data correctly, the function you should use is just COUNT. You would count the users whose start waitlist times are before the end of the current month, and end waitlist times are after the beginning of the current month.
I hope you can make sense of this. If not, it would help me if you could attach a sample packaged workbook, and I'll use it to try to find a solution. Let me also know if your data source would allow you to do a self-join.