- declaring `adminUsernames` and then using `userNames` (but I assume that's not what you're talking about because that won't even compile)
- you're making an array with its length N instead of length 0 capacity N
(I totally agree with your point, I just like looking for bugs)
But... also it's a mistake that's not even possible in the Ruby example due to not having to handle the "irrelevant" internal details of appending to the array, so maybe there's a point to it after all.
var names []string
for i, user := range users {
if user.isAdmin {
names = append(names, user.Name)
}
}