If your relationship between the main layout table and the portal table is correct you shouldn't need to create portal rows at all. You would have other fields in the portal row that need to be filled, but the portal itself simply displays records from a related table.
I have little experience but I have never seen portal rows create themselves.
Whatever the quality of the Relationship, there must be precise steps and conditions to follow.
If this is of intermediate or expert level, I will keep on doing it manually.
What Rick is suggesting, I think is that a relationship using the marital status could link automatically to all 30 participants record with no need to create any records.
But I suspect that doesn't fit what you need to happen in your database.
Could it be that you have a many to many relationship with a join table between Activities and Participants and that you are trying to create a set of records in the Join table that link a specific group of Participants to the current Activity?
If so, such can be scripted.