From dplyr issue 2916.

The following appears to work.

suppressPackageStartupMessages(library("dplyr"))

COL <- "homeworld"
starwars %>%
  group_by(.data[[COL]]) %>%
  head(n=1)

## # A tibble: 1 x 14
## # Groups:   COL [1]
##             name height  mass hair_color skin_color eye_color birth_year
##            <chr>  <int> <dbl>      <chr>      <chr>     <chr>      <dbl>
## 1 Luke Skywalker    172    77      blond       fair      blue         19
## # ... with 7 more variables: gender <chr>, homeworld <chr>, species <chr>,
## #   films <list>, vehicles <list>, starships <list>, COL <chr>

Though notice it reports the grouping is by “COL“, not by “homeworld“. Also the data set now has 14 columns, not the original 13 from the starwars data set.

And this seemingly similar variation (currently) throws an exception:

homeworld <- "homeworld"

starwars %>%
  group_by(.data[[homeworld]]) %>% 
  head(n=1) 

## Error in mutate_impl(.data, dots): Evaluation error: Must subset with a string.

I know this will cost me what little community good-will I might have left (after already having raised this, unsolicited, many times), but please consider using our package wrapr::let() for tasks such as the above.

library("wrapr")

let(
  c(COL = "homeworld"),
  
  starwars %>%
    group_by(COL) %>%
    head(n=1)
)

## # A tibble: 1 x 13
## # Groups:   homeworld [1]
##             name height  mass hair_color skin_color eye_color birth_year
##            <chr>  <int> <dbl>      <chr>      <chr>     <chr>      <dbl>
## 1 Luke Skywalker    172    77      blond       fair      blue         19
## # ... with 6 more variables: gender <chr>, homeworld <chr>, species <chr>,
## #   films <list>, vehicles <list>, starships <list>

let(
  c(homeworld = "homeworld"),
  
  starwars %>%
    group_by(homeworld) %>% 
    head(n=1)
)

## # A tibble: 1 x 13
## # Groups:   homeworld [1]
##             name height  mass hair_color skin_color eye_color birth_year
##            <chr>  <int> <dbl>      <chr>      <chr>     <chr>      <dbl>
## 1 Luke Skywalker    172    77      blond       fair      blue         19
## # ... with 6 more variables: gender <chr>, homeworld <chr>, species <chr>,
## #   films <list>, vehicles <list>, starships <list>

Some explanation can be found here.