When you add a new member or via the member list management area, you can add notes and a followup date to each member. In the members list, you can use the Order dropdown, to filter for followup dates only.
Customer notes can be added when new members are added, via the bulk actions form on members list, or via the member details popup window. A full history of all notes is kept. Use followup dates for best effect
Use member status and member groups to give your members tags, that can later be used for searching, targeting newsletters or password protecting pages for select user groups. Each member can also be assigned a price group.