Nested if and max if statement in R error: unused argument

Still quite new to R and I would like to identify the biggest value of a variable within a group when an other variable is postive/negative. Specifically, if l_diff<0, I would like to find the biggest value of t in each group when pos_n<0. If l_diff>0, I would like to find the biggest value of t in each group when pos_p>0. Example data:

l_diff <- c(-1,-1,-1,-1,-1,-1,1,1,1,1,1)
pos_n <- c(2,2,1,-4,-2,-2,2,1,-5,4,8)
pos_p <- c(3,4,-5,6,-7,2,-3,3,2,1,4)
t <- c(5,7,3,1,6,2,7,5,3,2,1)
group <- c(1,1,1,1,1,1,2,2,2,2,2)
db <- data.frame(cbind(l_diff,pos_n, pos_p, t, group))

Desired output:

cmax<- c(6,6,6,6,6,6,5,5,5,5,5)

I tried the following:

db<-db %>%
  group_by((group)) %>%
  mutate(ifelse(l_diff<0, t1 = max(t[pos_n<0], ifelse(l_diff>0, t1 = max(t[pos_p >0])))))

But I get the following error:

Error: Problem with mutate() input ..1. x unused argument (t1 = max(t[pos_n < 0], ifelse(l_diff > 0, t1 = max(t[pos_p > 0])))) i Input ..1 is ifelse(...). i The error occurred in group 1: (group) = 1.

Any idea what may be wrong or any other suggestions?