Add new database table to website software. Configure this table so email templates may contain data from this table. Result:
New members fill out online application; then software sends a 'welcome email' (already configured) that will include data fields from the new table. No member will get the same row of data.
NOTE: the existing software has email templates that contain data from the application. ie: Welcome, [[first name]], etc.
The programming must configure the system to 'assign' a row of data from your new table - to each new member - and will not assign the same data twice.
After testing, the programmer will fill the new table with data; backup customized software provide to owner.