Unknown User Reference on Folder Security

I have found 6 unknown users on one of my folder. The Icon is a question mark (?) and face profile. The names are all like this:

S-1-5-21-343818398-1390067357-839522115-500

S-1-5-21-343818398-1390067357-839522115-512

S-1-5-21-343818398-1390067357-839522115-513

S-1-5-21-343818398-1390067357-839522115-515

All the same except the last 3 digits. Anyone know why they are there and is it safe to assume removing them will not harm anything?

Thanks

Grajek