question on random assignment

Hi,
I had a question on randomly assigning participants to two experimental
groups when you are running an ongoing study and you don't know how many
participants there will be in the end.
For instance, with this Random Assignment tool for factorial experiments, it asks how many participants you have.

have.http://methodologymedia.psu.edu/most/rannumgenerator

How would you do this if the total number of participants is undetermined and the study is ongoing.
Thanks for any help,
Floyd