When she’s done, she adds it to the Campaign layout:

Next, she creates a new trigger from Setup-->Customize-->Campaigns-->Campaign Members-->Triggers. 

With some help from the Salesforce Community, she copies and pastes the code below and saves the trigger, and she’s done!

trigger UpdateCmpRegistered on CampaignMember (before update) {

    Campaign c = [select Id,Registered__c, name from Campaign where Id = :Trigger.new[0].CampaignId limit 1];

    for(CampaignMember cm : Trigger.new){

        if (cm.Status == 'RSVP-Yes') {

          if (c.Registered__c== null) {

              c.Registered__c=0;

              }

          c.Registered__c += 1;

        }

     }

    update(c);

}

 The result looks like this:

For inquiring minds, here’s what’s happening at each line in English:

1)      Run the trigger called UpdateCmpRSVP on all campaign members that are updated

2)      Find the ID of the campaign for these campaigns

3)      Loop through all the campaign members that were updated

4)      If the Status = RSVP Yes, do the rest of the stuff, otherwise,s do nothing

5)      If the Campaign custom field doesn’t have a value, then do the stuff on the next line

6)      Set the campaign field RSVP_Yes__c to 0 if it was null in line 5

7)      Signals the end of stuff to do for the line 5 criteria

8)      Now add 1 to the campaign field RSVP_Yes__c

9)      Signals the end of stuff to do for the line 4 criteria

10)   Update the campaign

11)   Signals the end of the trigger

Note that this trigger isn’t best practice, and though it works, the triggers you create should generally have more checks to test to check for bad results, such as if the RSVP_Yes__c field doesn’t exist as well.