Note: Before you begin, make sure you already have a YouTube channel associated with a Google Brand Account. What's that? See the following: How do I create a brand channel on YouTube?
Sign in to YouTube
Sign into YouTube with your (company or orgnization) Google Account that has access to the channel. Click the profile image in the upper right and click Switch account so you can choose the Brand Account for the channel.
For this example, I'll use the YourCompanyAnniversary.com Brand Account (GS Marketing Group brand) and associated channel. After making the switch, the page refreshes and the profile image changes to the YourCompanyAnniversary.com logomark. I'll click the profile image in the top right again to reopen the menu—which looks very much the same.
This time, I'll click Settings to get to the Account Settings (Overview), seen here:
In the Account Information section, I'll click the Add or remove managers link to get to a Brand Account prompt.
Now you should see a prompt like this:
Note that this panel gives you the ability to manage users and their permissions for the Brand Account. Click the Manage Permissions button and a new panel will open in front of the other, showing a list of users already associated with the Brand Account. Click the icon (with a plus and silhouettes) in the upper right corner of the Manage permissions panel to add one or multiple new user(s).
Type the email address for each new user you wish to grant access. Click Choose a role and select Manager to allow the new user(s) to work with your YouTube channel. Then click Invite to send the invitation email to the new user(s).
That's it. The rest is up to each invited user, to click and accept the invitation in the email message.