When adding a website as a new project, step 5 of the process prompts you to add competitors. You can either add them manually, or get suggestions of competitors based on websites that rank for similar keywords as the project's website does:

You can add up to 10 competitors to a single project.

If your project doesn't already have competitors, there are 2 ways to add them:

Any project that doesn't have competitors will show the option to add competitors:

You'll also see this option if you try to open Rank Tracker > Competitors Overview for a project without competitors:
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Once you already have competitors, you can edit them in Rank Tracker > Competitors Overview by clicking on the "Edit Competitors" as shown:

Or anywhere that provides an option to go to the project's Settings.

Here's the way to do so from Rank Tracker > Overview:

Here's the way to do so from the main Dashboard, via the 3 dot menu on the right of any project:

Finally, click on the Competitors tab and add or modify your competitors accordingly.

Once you are done, click "Save".

You can click on "Show competitors" to quickly see their metrics against the chosen project's metrics in the dashboard:

To compare their performance over time​ against the chosen project's website, click on "Compare on Overview":

This will open up Site Explorer > Overview for the project's website and have it's competitors pre-loaded in the performance chart:

In this case we are seeing the Organic Traffic over time of help.ahrefs.com against 3 of its competitors. You can select a different metric from the dropdown, or add/remove competitors as you need to.

Once competitors are added to your project, you can go to Rank Tracker > Competitors Overview and view their positions in Google for the chosen project's Tracked keywords: