If I join the Companions, will that keep me from joining the Thieves Guild? Or the Dark Brotherhood?
Answer
Nope! There’s nothing preventing you from holding leadership positions in all 4 major guilds! (After you finish the respective questlines, that is)
Bard College and the Civil War quests are also fair game (though you do have to pick a side in the Civil War).
Attribution
Source : Link , Question Author : Pithlit , Answer Author : Raven Dreamer