Do you want to add "straight" lines for each person, or the exact data? If the latter, you could use:
library(lme4)
#> Loading required package: Matrix
library(ggplot2)
data("sleepstudy")
ids <- sample(levels(sleepstudy$Subject), size = 8, replace = FALSE)
sleepstudy <- dplyr::filter(sleepstudy, Subject %in% ids)
ggplot(sleepstudy, aes(x = Days, y = Reaction)) +
geom_line(aes(color = Subject)) +
geom_smooth(method = "lm")

Created on 2019-09-21 by the reprex package (v0.3.0)
Else, this code might be what you're looking for:
library(lme4)
#> Loading required package: Matrix
library(ggplot2)
data("sleepstudy")
ids <- sample(levels(sleepstudy$Subject), size = 8, replace = FALSE)
sleepstudy <- dplyr::filter(sleepstudy, Subject %in% ids)
ggplot(sleepstudy, aes(x = Days, y = Reaction)) +
geom_smooth(aes(color = Subject), method = "lm", se = FALSE) +
geom_smooth(method = "lm", se = FALSE, color = "black")

Created on 2019-09-21 by the reprex package (v0.3.0)