Greetings,
After adding your followups, on the Followups page click the "Reset Users" button. Set all users to be at "End of sequence" by setting Seq # to your last followup's Seq # + 1.
New users will then receive followups and old users won't. However, if you later add more followups ALL users will be in line to receive them (when due). For the "old" users that may mean next dailymail unless you use the "update all users on this followup to new delay" option when adding the followup.
Please let me know if you require further clarification or if you want to do something else.
Regards