set.seed(1)
 df1 <- data.frame(matrix(nrow=20,ncol=3))
 colnames(df1) <- c("group","trial","hour")
 df1$group <- rep(c("a","b"),each=10)
 df1$trial <- rep(c(1,1,1,1,1,2,2,2,2,2),times=2)
 df1$hour <- rep(c(1,11,21,31,41),times=4)


 df2 <- data.frame(matrix(nrow=4,ncol=3))
 colnames(df2) <- c("group","trial","end")
 df2$group <- c("a","a","b","b")
 df2$trial <- c(1,2,1,2)
 df2$end <- runif(4,1,40)

df2 %>%
   left_join(df1,by=c("group","trial")) %>%
   group_by(group,trial) %>%
   filter(hour<= end)

df2 %>%
   left_join(df1,by=c("group","trial")) %>%
   group_by(group,trial) %>%
   slice(1:n(hour<= end)+1)`

library(dplyr)
df2 %>%
  left_join(df1,by=c("group","trial")) %>%
  group_by(group,trial) %>%
  slice({i1 <- which(hour <= end)
        unique(c(i1, pmin(tail(i1, 1) + 1), n()))})