To create a Website Custom Audience, click on Audience from the left hand side menu. Click on the 'Create New Audience Types' button at the top right hand side of the screen.
Click on the "Website" icon to choose the type of audience you want to create.
Give your audience a name and then specify the rules you wish to use in order to add people to that audience.
If you are trying to add all site visitors you might set the rule as:
Property: contains any
In the next field, you can set the retention of the audience. This is the length of time over which people will be added to this audience. If, for instance, you set it as 30 days, users who have met the rule criteria you set anytime as far back as 30 days before today will be included in the audience. The maximum is 180 days.