I, myself, am an alter, and have been the host/main one out for around 10-11 years. Most of the people who know me in real life don't realize that, so I sometimes find it kinda funny when knowing that I have alters makes people nervous or scared of me, or they don't want to "deal" with an alter, even though they're "dealing" with me and I'm an alter.

Anyway, what I wanted to say was that I'm glad to hear that the idea of alters doesn't scare you, and it's completely understandable to be wary of alters you don't know yet.
The only time my alters have attempted to physically hurt/successfully hurt someone (other than myself) is when they were provoked. If someone tries to physically hurt them/me, then they're going to try to physically hurt that person in return. Otherwise, they'll express any feelings of dislike or anger or something verbally. Perhaps, at a place where you feel safer, you could try asking about the darker alters, to help you learn more about them and to understand them so they're not as scary to you. If not much is known about them, then just learn what you can about them so that if they ever come out around you, you'll at least feel a bit more prepared and will know a bit about who you're encountering and what to expect. If you don't know what to expect, then expect anything, and try to be as non-threatening as possible.
*Possible trigger, talk of violence*
My boyfriend Mike had a knife stuck against his throat by one of my alters, Kat, when she still went by the name Kataki. She hated him because she loved my ex and saw him as an intruder to her life who was in her way. But he remained sitting on the couch, calm, and did not let her provoke him into retaliating or anything. He was letting her know that he wasn't a threat, and that she had no reason to harm him; but he also showed no fear and was making it clear that her scare tactics weren't working. So she gave up on them, and never did hurt him.
Now, I'm not saying that everyone could remain calm and stuff with a knife to their throat, but it's just an example of how to stay non-threatening despite any scare tactics that alters can try to use. Obviously, you'll have your own "line of comfort" that you'll try to prevent from being crossed, but unless an alter attacks you first or does something like hold a knife to you, try to remain calm and un-threatening. You can even try talking to them, that often surprises darker or angry alters. They're not used to having people care about them or ask them questions such as "what's your favorite color", or even, "what's your name". Mike tried that with Kat/Kataki and it's what helped him to keep the situation under control and not have it escalate.
*end trigger*
As for your other friend, perhaps you could set up something like a password system? A newly surfaced alter of mine, Cassidy, likes to pretend to be Cassie, and she gets Mike to give her stuff like Cassie's ice cream. So Mike gave Cassie a password to tell him when he asked, and she was to keep it from Cassidy. It's helped him to know exactly who is out so far. Perhaps you could try something similar, either with a password or maybe a specific hand gesture or something.