If you need to tie an activity to a particular account for a contact, there are two ways to do so.
The first method would simply be to create an Activity for the contact that the account belongs to, from whatever area of the database you're currently in. On the Activity Details page, you'll see a Linked Accounts area where you can select one of their accounts to tie the activity to:
Just click the add button in that Linked Accounts area and select which of the client's accounts you want to link to the activity. Then, Save the Activity.
The second method is to edit an existing activity to tie it to an account. When you open up an Activity to update it, you'll see the same Linked Accounts area that we saw above when creating a new Activity.