Pass column names as strings to group_by and summarize

For this you can now use _at versions of the verbs

df %>%  
  group_by_at(cols2group) %>% 
  summarize_at(.vars = col2summarize, .funs = min)

Edit (2021-06-09):

Please see Ronak Shah's answer, using

mutate(across(all_of(cols2summarize), min))

Now the preferred option

Another option is to use non-standard evaluation (NSE), and have R interpret the string as quoted names of objects:

cols2group <- c("x","y")
col2summarize <- "z"

df %>%  
  group_by(!!rlang::sym(cols2group)) %>% 
  summarize(min(!!rlang::sym(col2summarize)))

The rlang::sym() function takes the strings and turns them into quotes, which are in turn unquoted by !! and used as names in the context of df where they refer to the relevant columns. There's different ways of doing the same thing, as always, and this is the shorthand I tend to use!

From dplyr 1.0.0 you can use across :

library(dplyr)

cols2group <- c("x","y")
col2summarize <- "z"

df %>%
  group_by(across(all_of(cols2group))) %>%
  summarise(across(all_of(col2summarize), min)) %>%
  ungroup

#   x       y     z
#  <chr> <dbl> <int>
#1 a         1     1
#2 a         2     3
#3 b         2     4
#4 b         3     5