Look up Replace Field Contents in the on line help file. This will do what you want and can be done manually or in a script. It has several different options, so it's a good idea to read up on it before you use it.
You can simply put a constant like "1" in all the fields of your current found set or use a more sophisticated option.
There are several options for building a text list of values from multiple records while separating the values with carriage returns. Learning these tricks can be very useful to the developer since these fields can enable very powerful relationships.
Here's the method I prefer:
In your case, let's call your Field from step 1, EmailGroup and make it a number field.
Create a self join relationship linking EmailGroup to EmailGroup of the same table.
Call the new Table Instance "SameGroup".
Now define a value list with the following options:
Specify Field: OriginalTablename:: PlayerName
Show only related values: SameGroup
Name this value list GroupNames.
Now the function: ValueListItems ( Get(fileName) ; "GroupNames" )
Will return the list you want of all players with the same email group number as the current record.
You can use this expression in a calculation field that returns text or use it in a Set Field step to assign the names to a global field.
Thanks for the help PhilModJunk. Your solution for the value list works well.
The methods described have their draw backs also.
Replace is not a "safe" method to use if the records it tries to change are being edited. If a user has their cursor in a record's field, it's locked and Replace will skip that record and tell you that one or more records were locked, but you have no way to know which records didn't get the update.
ValueListItems() will return an empty string if either the filename or the value list name is even one character different from their actual names. Get(Filename) let's you avoid problems if you change the name of your database file, but if you change or mis-type the name of your value list, you won't get an error message, just an empty list.
Still, it's elegant and it works, you just have to be careful.