R: count distinct IDs where selected column is non-null

Question

I have the following dataframe:

user_id <- c(97, 97, 97, 97, 96, 95, 95, 94, 94)
event_id <- c(42, 15, 43, 12, 44, 32, 38, 10, 11)
plan_id <- c(NA, 3