Can I add people to a group based on when they responded to a poll?

No, at this time, we do not support automatically filtering participants into groups based on when they texted in, or when they offered a particular response (e.g. respondents from August 2015; students who said they were high school seniors in 2014). This is a popular request and we're tracking it closely.
In the meantime, our support team may be able to help manually configure the groups you need. Please contact us at support@textizen.com.