Hello,
wakecarter’s sequential participant id will work. When recruiting using SONA you could use a code generated on SONA to assign participants to different groups. And yes you need some code to determine the group assignment on the basis of the participant id. Notice that these approaches do not take into account that participants might not finish the experiment. Therefore the participant count per group might be off. If you want an even distribution you might want to take a look at the vespr-tool
It is possible to have different instructions based on the group assignment.
All can be done in one experiment aka psyexp-file.
Best wishes Jens