Random Assignment question

[Floyd Patterson]

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
 

[PokePsych]

Some software (e.g., qualtrics) has the option of random assignment (it has an option randomly show between these two categories and show each condition equally).