Triangulation, Blocking Friendships

Seems like every time you talk to someone else, the person in your life you suspect has Narcissism has to insert themselves into the conversation, or literally get between you, or cut you off? Do they seem to get upset whenever you talk to someone when they weren't present, especially someone they know or like, or someone they're apparently intimidated by or jealous of?

(Notwithstanding you speaking to a person who has hurt or betrayed them. That's something that Narcissists will often do; make sure to preserve a relationship with a person who has betrayed, disrespected, or abused their target, and will often do so as if to support and sympathize with the perpetrator, regardless of the original relationship between them. Preserving peace in a family is one thing; being "buddies" and sympathizing with an acquaintance, a "friend", an ex, or family member who treated the target abusively is quite another.)

Or they act hurt or offended, like you're leaving them out on purpose every time you speak to someone else in a friendly manner, or when someone speaks to you?

Triangulation is the name of the game. Because of their paranoia and control issues, they have to control ties between people they know, and communications, and relationships.

If you become "close" with someone else, then THEY might get left out or abandoned by either you or the other person (in their mind).
THEY might become the target of the same things they're already doing to others (or to you) if you make some kind of connection with another person.

Since they control and manipulate ties and relationships between those around them, especially targets, they are likely to fear that they are going to be the one being manipulated or abandoned if they aren't in control of a connection between you and someone else.

If you are a person who has good intentions and healthy relationship practices, you wouldn't triangulate against them, betray them, or abandon them in the first place just because you had a friend.
(If that's not the case, then they might not be paranoid after all.)
But assuming that you do not do those things, and would not to them, then their actions regarding your connections with others has to do with projection, paranoia, or control, or all of the above.