OK, I figured it out!
For anyone else trying to accomplish this:
When you click on "Manage Database Fields" in the settings, you get a screen with tabs that include "Participant Info" "Personal Info" "Administrative Info" "Record Info" etc. I named my primary contact fields under "Participant Info," and for each of those fields under the "Type" column, I set them to "main" (as in main group. That is the default, I believe). Those are my primary contact fields.
Then going to the "Field Groups" tab, I added another field group called "Firm Members." This gave me another tab in the line-up called (no surprise) "Firm Members." There I added a field titled "Company."
So now I have tabs for the field groups "Participant Info" and "Company."
So if I want the following output:
Individual Membership Page:
- Member Name
- Phone
- Email
Firm Membership Page:
- Company Name
- Member Name
- Phone
- Email
My shortcode for these pages would be:
Individual Membership Page: [pdb_signup groups="main"]
Firm Membership Page: [pdb_signup groups="main,firm_members"]
This adds the "Company Name" field to the Firm Membership page, because that field is in the firm_members group.
It might be helpful if I also explained that there is a difference between "Name" and "Title": "Title" is what you call your field; "Name" is what the plug-in calls it, based on the title you give it. Same for the Field Groups. I titled my field group "Firm Members," and the plug-in named it "firm_members." It's the plugin-generated name that you need to use in the short code. Also, please make note that the Field Group titled "Participant Info" is named "main" in the drop-down "group" column. So any fields under "Participant Info" will be "main" in the short code.
I hope that helps anyone else who is trying to get different forms to feed into the same database.
I'm really looking forward to using this plug-in for our membership drive! Thanks, xnau!