a) We are not going to send any emails or texts to anyone from your contacts. Apps like Gather / IRL spam people for growth, and we would rather earn your trust and grow slowly.
b) You can use Millow without ever inviting anyone else on. It can be used in a 100% single-player mode, where it will do exactly what you said. You can set reminders for your close friends, and Millow will send you a push notification once a week (Sunday afternoon in your device's time zone) to remind you about the friends that you're set to meet this week (it won't send a push reminder if there are none).
c) Monetization isn't something we're particularly concerned with in the immediate future (we are a team of two, and have sufficient savings from the acquisition of my previous startup Screenhero by Slack), and we are doing this mainly because we believe in the social impact of what we're doing. We do have plans for future monetization (e.g. if we help you make a plan to meet up with a friend at a local restaurant, the restaurant pays us similarly to OpenTable).
Happy to chat more, here or via email (support@millow.app). We're a two person team, and are eager for learning more and improving.