I'm getting messages from user not in my friendlist

[roughnecks]

Hi.

I've been trading with scrap.tf bots recently and every time I want to buy something, their bot sends me the trade offer along with a text message, which I receive via bitlbee only.
The sender's nickname is composed by an underscore, followed by its steam ID.

How can they do that, if we're not friends on Steam?

[AiyionPrime]

https://steamcommunity.com/app/440/discussions/0/2765630416818067729/
They may message you by beeing in the same group.

[roughnecks]

I haven't joined any group.

[AiyionPrime]

And youre sure, youre not friends with any of their bots?
Like when you get gifts or trade and stuff you become friends automagically?

[roughnecks]

Yeah, I'm sure. BTW there's no need to be friends to trade and I don't think scrap.tf gifts anything.