Some firms have one HMRC user ID that all staff use. Some firms create a user ID for each member of staff and allocate clients to that user ID. The former is administratively easier but less secure and can be disruptive when a member of staff leaves (eg change the password). The latter is the better course of action but requires a lot of time to administer, especially in larger practices.
The ASA requires clients to be dealt with one at a time. We're starting our ASA so it's an opporunity for us to review our policy.
How do you manager your staff HMRC access?