You can assign Tags (both User and Generic Tags) individually or in bulk.
Assigning User Tags to Ad Accounts now also builds the relationship between Users and Ad Accounts to facilitate Users being emailed Alerts - see below.
The page to assign Tags is Settings / Tags / Assign Tags.
Assigning Tags to an Ad Account
There are multiple ways to assign Tags:
Add individually using the dropdown
Select some or all and use the 'Add Tags in Bulk' dropdown
Filter the results and use the 'Add Tags in Bulk' dropdown
Removing Tags from an Ad Account
There are multiple ways to remove Tags:
Remove individually using the 'X' in the tag
Select some or all and use the 'Remove Tags in Bulk' dropdown
Filter the results and use the 'Remove Tags in Bulk' dropdown