The Steam version has options for "private" servers where you have to be invited in via the friends list. It's possible that there will be some sort of similar integration in the GOG version once the Galaxy client is more complete.

Server Password, Voting Options, Change Race Ability, Kick Options: You would need to change the source code for the game entirely. Those were never features of the original game.

Playing with a group of friends: Just limit the number of slots for available races. You see where it says Maximum Predators, Maximum Marines, Maximum Aliens? Just figure out what your friends want to be, limit the max number of slots for each race to correspond to what your friends want to be, and have everybody connect at the same time. That way no one else will be able to join you. It also helps to title servers as PRIVATE or something to that effect.

yeah that's not a bad idea, thanks

At least some of these options could be implemented with almost no changes to the original game. A kick and a ban command could be added to the console. The only change in the original game would be to recognize these commands and forward them to the multiplayer part, which is developed or adapted by GOG anyway.

Not necessarily. It depends on whether or not connectivity runs directly through the third party client or is P2P. There was in fact a mod for AVP Gold Edition that allowed for kicking and banning of players through multiplayer (search MelPyth mod if you're interested). The source code to AVP Classic 2000 is different from Gold Edition and Rebellion doesn't want to release it. You also have to remember that this game is 15 years old and implementing such a system may be seen as not more trouble than it's worth by the developers. It's also not GOG's responsibility or prerogative to make these adaptations to the game's coding, regardless of how easy it would be to add if they did have the source code.