To do this you'll probably want to set up two membership items. The first membership is your standard membership, which is offered to the general public. For the sake of this example, let's assume your standard membership is $20 a month.
Next you will need to create a membership item for your current clients, you may want to call it something like "Preferred Client Membership." This will differentiate the discounted membership from your standard membership and make your current clients feel special. For the Preferred Client Membership you are going to set a trial 1 period price of $10 (50% discount) for at least 1 month. You will then set a regular period price of $20 per month.
After the trial 1 period is over, your current clients (subscribed to the Preferred Client Membership) will be charged the standard rate of $20 per month.
Now, that you have set your membership items up, you need to put a button tag for your standard membership in a public article on your site. This article will be linked to through your menu.
For your preferred client membership, add a button tag to your promotional article for the preferred client membership. In the article parameters (in the right column) you will see two sets of dates. One date governs when the article will start being published and one sets when to finish publishing the article. The start publish date will already be set to start immediately after saving the article. Set the finish publish date for when you want to stop offering the Preferred Client Membership.
Now all you need is the URL to the Preferred Client Membership article page. The easiest way to find the URL is to create a menu link to the article. However, you want to set "published" to "no". This way the menu item will not be published on your site when you click save. After saving the menu item, take at its details. One of the first details you will see is called "link". This is the link to the article. Copy the link from the input box. Test it in your browser's address bar to make sure it takes you to the Preferred Client Membership promo page.
Now, your all set. You can distribute the link to your current clients. Make sure to let them know that it is a limited time offer. In reality, the Preferred Client Membership membership item will remain active in the Nice Membership extension. This is needed for the extension's proper management of the members. However, because the promotional page will finish being published at some point, people will not be able to sigh up for the membership, because they will not have access to the subscription button.
~ All the best