How can I make a COUNTIF(or similar) that counts the amount of people join within a time period?
You do not really need any regex function. You can use the different versions of the following formula instead.
- To find all values within the second month (February)
=ARRAYFORMULA(COUNTIFS(V21:V38,"<>",MONTH(U21:U38), "="&2))
- To just find the
Y
values within the second month (February)
=ARRAYFORMULA(COUNTIFS(V21:V38,"Y",MONTH(U21:U38), "="&2))
Have a look at the example I created in your sheet.

How the formula =ARRAYFORMULA(COUNTIFS(V21:V38,"<>",MONTH(U21:U38), "="&2))
works:
COUNTIFS
looks into the first range U21:V21
and in cell V21
it calculates the value according to the criterion "<>"
which means not empty.
It then goes to the next cell U21
where the function MONTH
has already extracted the month from cell that in our case is different from the second criterion 2
and in this case COUNTIFS
drops the value.
Then ARRAYFORMULA
takes over and repeats the process in the next row and keeps on doing that till it hits the end of our range on row 38.
The final result is 6, which means only 6 pairs met both the criteria.
Functions used: