Dear 100 Hour Board,
Is there an app that will help me compare my schedule with other's schedules, to find a time when we can get together as a group?
-When are you available?
Dear Sorry, I'm taken,
I second guppy's answer for WhenisGood.
Personally, though, I prefer Doodle Polls. There's an app on Google Play (so I would assume there's one for iOS too) for it, and I think the interfacing is really nice.
Additionally, using Slack if you're going to be working with this group frequently. It's a great business/work/group messaging system that you can use to communicate the project and schedule if you need to.
WhenIsGood is the website my classmates and classes always used to schedule stuff. It's not an app but the website is works pretty well.
-guppy of doom