set.seed(1)
data <- data.frame(g = LETTERS[1:2], x = runif(2))
f <- function(x, g = NULL) {
    x |> 
        dplyr::group_by(dplyr::across(dplyr::all_of(g))) |> 
        dplyr::summarise(mean_x = mean(.data[["x"]]))
}
f(data)
#> # A tibble: 1 × 1
#>   mean_x
#>    <dbl>
#> 1  0.319

data |> 
    dtplyr::lazy_dt() |> 
    f()
#> Source: local data table [1 x 1]
#> Call:   `_DT1`[, .(mean_x = mean(x))]
#> 
#>   mean_x
#>    <dbl>
#> 1  0.319
#> 
#> # Use as.data.table()/as.data.frame()/as_tibble() to access results

f(data, g = "g")
#> # A tibble: 2 × 2
#>   g     mean_x
#>   <chr>  <dbl>
#> 1 A      0.266
#> 2 B      0.372

data |> 
    dtplyr::lazy_dt() |> 
    f(g = "g")
#> Source: local data table [2 x 2]
#> Call:   `_DT2`[, .(mean_x = mean(x)), keyby = .(g)]
#> 
#>   g     mean_x
#>   <chr>  <dbl>
#> 1 A      0.266
#> 2 B      0.372
#> 
#> # Use as.data.table()/as.data.frame()/as_tibble() to access results