Create vector from split dataframe elements

I am using igraph and I would like to use the mark.groups function to specify some groups within the network. The function requires groups to be specified by vectors of numerical Node identifiers - e.g.mark.groups = c(1,5,7).

I have a list of groups and associated in a dataframe format:

groups
Group_ID Node_ID
1        1
1        2
1        3
2        4
2        5

Rather than type these by hand in the mark.groups function, I split the dataframe into a list of sub-dataframes:

members <- groups %>%
   group_split(Group_ID, keep = FALSE)

I am then trying to refer to these elements when plotting the chart. This code works:

plot(net,
   mark.groups = c(members[[1]], members[[2]])
)

But I would like to be able to refer to all the split-out elements without having to type each individually. I have tried this, but it throws a Error in members[[1]]:members[[2]] : NA/NaN argument error:

plot(net,
   mark.groups = c(members[[1]]:members[[2]])
)

I thought it might be possible to create a list of the names of the elements using this:

mem_count <- length(members)
mem_list <- paste("members[[", 1:mem_count[1], "]],", sep = "")

and then refer to them in the plot() function:

plot(net,
   mark.groups = memlist
)

But that doesn't seem to work either, giving this error Error in simple_vs_index(x, ii, na_ok) : Unknown vertex selected.

Any ideas much appreciated.