df<-data.frame(DateTime=c("2016-08-01 12:04:07","2016-08-01 12:06:07","2016-08-01 12:06:58","2016-08-01 13:12:12","2016-08-01 14:04:07","2016-08-01 13:12:45","2016-08-01 15:04:07","2016-08-01 17:13:16","2016-08-01 17:21:16","2016-08-01 17:21:34","2016-08-01 17:23:42","2016-08-01 17:27:16","2016-08-01 17:27:22","2016-08-01 17:28:01","2016-08-01 17:29:28","2016-08-01 17:28:08","2016-08-01 17:28:15"),
               Observer=c("Peter","Sophie","Peter","Peter","Sophie","Sophie","Peter","Sophie","Sophie","Sophie","Peter","Peter","Peter","Andreu","Sophie","Anna","Peter"),
               Animal=c(1,2,1,1,2,1,2,1,2,1,1,2,2,2,1,2,2))
df$DateTime<- as.POSIXct(df$DateTime, format= "%Y-%m-%d %H:%M:%S", tz= "UTC")

df
              DateTime Observer Animal
1  2016-08-01 12:04:07    Peter      1
2  2016-08-01 12:06:07   Sophie      2
3  2016-08-01 12:06:58    Peter      1
4  2016-08-01 13:12:12    Peter      1
5  2016-08-01 14:04:07   Sophie      2
6  2016-08-01 13:12:45   Sophie      1
7  2016-08-01 15:04:07    Peter      2
8  2016-08-01 17:13:16   Sophie      1
9  2016-08-01 17:21:16   Sophie      2
10 2016-08-01 17:21:34   Sophie      1
11 2016-08-01 17:23:42    Peter      1
12 2016-08-01 17:27:16    Peter      2
13 2016-08-01 17:27:22    Peter      2
14 2016-08-01 17:28:01   Andreu      2
15 2016-08-01 17:29:28   Sophie      1
16 2016-08-01 17:28:08     Anna      2
17 2016-08-01 17:28:15    Peter      2

df
              DateTime Observer Ind Other_observers         Who
1  2016-08-01 12:04:07    Peter   1               0          NA
2  2016-08-01 12:06:07   Sophie   2               0          NA
3  2016-08-01 12:06:58    Peter   1               0          NA
4  2016-08-01 13:12:12    Peter   1               1      Sophie
5  2016-08-01 14:04:07   Sophie   2               0          NA
6  2016-08-01 15:04:07    Peter   2               0          NA
7  2016-08-01 17:13:16   Sophie   1               0          NA
8  2016-08-01 17:21:16   Sophie   2               0          NA
9  2016-08-01 17:21:34   Sophie   1               0          NA
10 2016-08-01 17:23:42    Peter   1               0          NA
11 2016-08-01 17:27:16    Peter   2               2 Andreu Anna
12 2016-08-01 17:28:15    Peter   2               0          NA
13 2016-08-01 17:29:28   Sophie   1               0          NA