johnmurch's comment has a lot of what I'd say. I'm not so sure about his #2 or 3 but his #1 was really good.
I'd just price it super low and have the option to prepay for a year for a small discount. If I had built this, I wouldn't be having visions of raking in millions but you could realistically make a nice side income from it. If the price is like $4 a month or $20 for a year and you're basically just hosting a CRUD app that only stores text then it shouldn't be very expensive to run. Consider too that you built it just for fun. Put that all together and even if you charged $1 for life from every user you've made a profit. Maybe I'm wrong but that's how I think of my projects. It's like I was going to rent the servers and build something anyway so even if I make a single cent then I'm in the green.
But to answer your question more directly, yes. If this were a beautiful site/mobile app that did this one thing really well I would pay. It would be really pretty to look at (like Cheddar is/was), would be super convenient (open a URL or tap a phone icon), and do one thing well (I managed different todo lists with others and I didn't have to fuck with settings or signing in more than once a month or click/tap through more than one screen to get what I wanted) so it would be worth the few bucks I'd probably spend on a coffee or something.
A problem I see is getting groups to use it. Individuals all have their own preferred ways of todo-ing. A thought just came to me that if you made this awesome to work with alone then people would end up using it on their own and then they'd get together and use the group features later. Just a thought.
Tie-ins with other products would be good too. I don't know if they're monetizable but they're certainly great for promotion. If you could build an API and hook your app up to someone else's and vice versa that could definitely be a value-add for a lot of people. Throw up a payment form with Stripe and just see if people will pay $1 for fully refundable 30-day trial and the promise of getting in on some future features of your choosing. That will tell you a lot about how interested people are and if its worth exploring monetization.