The easiest way to give your users access to Duplicate Check is with the DC Permission Set. The advantage of using the Permission Set is that it is updated automatically whenever new classes, tabs etc are added to DC. This means you don't have to make any changes after future updates where new classes might be introduced. 

However, with the DC Permission Set users have access to all of Duplicate Check, including the settings in DC Setup. If you don't want your users to access certain DC pages, such as DC Setup or DC Job, make sure to deny them access to those pages. 

Alternatively, you can grant your users user access via profile settings‍ or a custom permission set instead. Note that profile settings and custom permission sets need to be updated manually whenever a new class, tab etc. is added to Duplicate Check.

Grant access with the DC Permission Set

To grant users access to all of Duplicate Check:

1. Go to Salesforce Setup > Users > Permission Sets.
2. Click the Duplicate Check for Salesforce permission set.
3. At the top, click Manage Assignments.
4. Click Add Assignments.
5. Select the users you want to assign the permission set to.
6. Click Assign.

You have now granted full access to Duplicate Check, including all classes for current and future DC releases, to the selected users.

To deny access to certain DC pages:

1. Go to Salesforce Setup > Users > Profiles.
2. Find the profile you want to edit.
3. On the profile, go to Object Settings.
4. Find the DC tabs you want to hide and edit. 
5. Under Tab Settings, select Tab Hidden.
6. Click Save.