count positive negative values in column by group

You need sum():

df %>% group_by(id) %>%  
  mutate(pos = sum(ff>0),
         neg = sum(ff<0))

For a fun (and a fast) solution data.table can also be used:

library(data.table)
setDT(df)    
df[, ":="(pos = sum(ff > 0), neg = sum(ff < 0)), by = id]