This is a good question - my usual repsonse is to use strsplit, then unnest and spread, which is also not super efficient:
library(dplyr)
library(tidyr)
dat %>% mutate(to = strsplit(to, ",")) %>%
unnest(to) %>%
group_by(from) %>%
mutate(row = row_number()) %>%
spread(row, to)
Source: local data frame [4 x 5]
date from 1 2 3
(time) (chr) (chr) (chr) (chr)
1 2015-10-22 15:03:17 person1@gmail.com person2@yahoo.com person3@hotmail.com NA
2 2015-10-22 15:03:17 person2@yahoo.com person3@hotmail.com NA NA
3 2015-10-22 15:03:17 person3@hotmail.com person4@msn.com person1@gmail.com person2@yahoo.com
4 2015-10-22 15:03:17 person4@msn.com person1@gmail.com NA NA
