Any investment in time and funds you made to building your team is worthwhile. This sort of customized attention will elicit a positive reaction from your PSA who will believe you care for him/her as a person and not just a stepping stone to your parochial interest. It will also depend on the amount of time you have at your disposal. All things considered, a personalised welcome letter using information from the affiliates profile is far appreciated than sending a mass automated mails to them.