Many activities calculates an Activity ID = Title & Date & Time
Many Contacts calculates a Contact ID = First Name & Last Name
You would be much better off using an auto-entered serial number or auto-entered Get (UUID) for your two ID fields. The calculated ID's that you are using can create problems for your database that are avoided by using one of these options for an auto-entered ID.
An Activity ID is calculated in Tutu's House Activities I would like the Activity ID to magically appear in the Who's Involved join table. The magic could happen once a month. Can this be done? How do I make it so?
For a many to many relationship like this, it makes no sense to do that. Creating a record in Who's Involved with a specific Activity ID is not needed except for that specific group of contacts that will be involved in that activity. This can happen automatically as part of the process of selecting specific contacts for a given activity. You need not have it happen on a scheduled basis as this would populate the join table with many unused and disconnected join table records.
The simplest way to manage a many to many relationship is to use a portal to the join table. For example, you can put a portal to Who's Involved on your Activities layout and then can use the Who's Involved::Contact ID field to select contacts for that activity. If you enable "allow creation of records via this relationship" for Who's Involved in the Activities to Who's Involved relationship, simply selecting a contact for this field will automatically copy over the Activity ID field's value and link the new join table record to the current Activity record.
You can also turn this around and place a portal to Who's Involved on the Contacts layout and use the Who's Involved::Activity ID field to select activities for a given Contact record.
Here's a demo file which coincidentally links Contacts to Events. You can examine it to see a working example of what I am describing as well as some examples of other tools for working with a many to many relationship: https://www.dropbox.com/s/oyir7cs0yxmbn6i/ManyToManywDemoWExtras.fp7
If you are using FileMaker 12 or newer, you can use Open from the File menu to open this file and produce a copy converted to the newer .fmp12 file format.
Mahalo Phil. Your "simplest way" looks like a good fit.
I'll have to learn how to create portals etc.
Thanks for the guidance.