Does joining one of these "guilds" preclude your joining the other ?

No, but you get a little extra dialouge with one of the members if you've finished the Companions.

You can join any and all guilds if you so choose. There is a bit of connection between the DB and the companions but its only one dialogue sentence; then there's Thieves Guild dialogue, its not much but there is still some intertwining of meeting. Its all dialogue though.

The only factions you can join that excludes another are the two civil war factions, they are mutually exclusive, though you can change sides at one point. There's also a path you can take that excludes the DB factions, but this is just 1-2 missions, and you got to "turn down" joining DB for it to open.

Can someone spoiler tag what is that sentence? I seem to have missed it even I got 500hours lol

You can join all guilds


All changes in the guild don't happen there is only a dialogue sequence

No, but it's odd being the Listener/Guild Master/Archmage/Harbinger.

Unfortunately, this is one of skyrim's biggest flaws. Joining factions has No affect on game play or npc responses.

Or being in the DB yet joining the Legion.

So whats the extra dialogue?

Thanks for the help.

Hmmm anyone?

I don't remember what was said specifically but that was what they talked about.

I opened the CK and found this :
There's nothing to it, Arnbjorn will just comment on you being a werewolf:

I might ask the same of you. For you are more than mortal. You are moon-born. You are wolf.
So you're a Companion, then? I can't imagine you got your gift anywhere else. I hope you have better luck with them than I did.
Oh yes, I was once brother to the Companions. Let's just say they found my methods "unsettling." The Dark Brotherhood, obviously, feels differently.