VoteWithMe and OutVote both can sync with your contacts and tell you those peoples’ voting histories.
The apps don’t know anything that isn’t already public information, so they can’t tell you how someone voted. What they do is collect information from publicly available records and present it all in one convenient location. You can see someone’s political affiliations and whether or not they voted in previous elections.
Both apps were designed to increase voter turnout using social pressure. According to VoteWithMe, reminders from family and friends are 20 times more effective in getting people to vote than the more traditional method of knocking on doors. That’s why the apps allow users to send prewritten messages like “You gonna vote?” to their contacts.
There are privacy elements to take into consideration with these apps before you download. According to The Verge, OutVote harvests personal data like company names and email and physical addresses. There’s also the awkwardness that might ensue if any of your friends’ affiliations take you by surprise.