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Brannel will appear as a guard near the front door to Mae'Var's former guildhall when it becomes your Thieves' Guild Stronghold, if Gorion's Ward is a thief. He'll prevent any member of the party from entering the first time you return to the guildhall after accepting it as your stronghold, if Gorion's Ward isn't within site. Once he speaks with the Ward, things will smoothed out and permanent access will be granted.

After your interaction with the shadow thief Ama while getting acquainted with your new guild and some of its members, he'll apologize for letting her barge in, saying that the signet she carried prevented him from keeping her out. He'll also says that his brother in the town where she claims to be the Shadowmaster of its local guildhouse has never made mention of her.

You're with <Gorion's Ward>? Well, send the danged fool over then. I needs to see <him/her> before I lets you in.

— first time reaction to party members

Sorry about letting in that Ama lady, boss. She just barged right past, and I didn't want to bounce her because of that signet she was carrying.

Funny thing is, I got a brother out in Esmeltaran that smuggles the odd turnip, and he never mentioned any Shadowmaster named Ama. Ain't the thing you talk about in public, I guess.