I use "people tags" (in my app) - serving a similar purpose as an Agenda list for each person. (And I have tags for regular contexts as well; everything actionable on one consolidated Next list, filterable by tag).
Often enough, when I bump into somebody or am speaking with someone on the phone, I remember to check (filter) for tasks that require this person's assistance/presence. It works well. I also use more generic tags for "types of people", e.g. Supplier, Client etc for less frequent contacts.
If the Agenda items are for a particular meeting, I put them as comment lines in a task that represents the meeting itself.