Which Contacts have been active in the last 90 days?

You can create Groups based on a relative date or time range. In your instance, that would be in the past 90 days. Use "Last Activity" as the value you're looking at. Last Activity is based on actions that contact did such as:

• Opening an email
• Clicking an email link
• Visiting a tracked page
• Logging into an integrated WordPress site
• Filling out a form (order or opt in form)

Here's an example of what your Group would look like. You can use any date range to make this most useful for your business!