You can make announcements to your members by adding a special box on member profile pages. The message will be displayed above the status bar, top center. Only you can control the content of the special box.
UPDATE: You can also add Text Box below Sign-In Box - - see example below.

INSTRUCTIONS:
1. Edit the text below, using your custom text message.
2. Add the entire block of code to the Custom Code Box (Analytics) via My Network/Tools/Custom Code
Note: If you sign out, it may take a while for the caching to catch up, and for the box to show. This is true for any tweak you make to your Ning Network.

WARNING: As in all tips offered on JenSocial, tips are hacks. Add at your own risk. Please be sure you do not leave out any characters, especially the ending closing paren and semi-colon.

CRITICAL TIP: If you include an apostrophe like in a statement: "Here's the trick." this code will break. The single quote or apostrophe breaks the flow of the DIV that wraps around your HTML. And if you really need to add a single quote within your text, please see TJ's explanation here. TJ is the Creator of jQueryHelp, and is an expert at jQuery.


<!-- Add Message to Profile Pages -->
<script type="text/javascript">
x$(document).ready(function() {
x$(".xg_widget_profiles_profile_show .xg_2col h1").after('<div style="font-size:12px; background-color:#FFFFFF; color:#000000; margin-top:15px; margin-bottom:15px; border:1px #333333 solid; padding:5px;" class="xg_module">To add a custom message box on member profile pages, change this text, and add the code to your Custom Code Box (Analytics). If you add your custom text here, it will be displayed on member profile pages directly above the Status Box. You need to keep this text simple. Be sure you do not use single quote marks within your message. If you want, you can add simple links like this: <a href="http://jensocial.com">Visit JenSocial</a> This is a great way to make announcements to your members.</div>');
});
</script>

How to Change the Font Size and Color:
Change the font size and color by editing this code, within your code block, above.
style="font-size:12px; color:#000000; background-color:#FFFFFF;
Also: As Garfield suggested, you may want to force a background color, in case your members have a background color where your text will not show.

Tip: If you make the font-size large, you may want to increase your padding.

Please let me know if you experience any issues with this tip. I tested it, but not as much as I would like.

Example:

Example Code with Image/Link:
<!-- Add Message to Profile Pages -->
<script type="text/javascript">
x$(document).ready(function() {
x$(".xg_widget_profiles_profile_show .xg_2col h1").after('<div style="font-size:12px; background-color:#FFFFFF; color:#000000; margin-top:15px; margin-bottom:15px; border:1px #333333 solid; padding:5px;" class="xg_module">To add a custom message box on member profile pages, change this text, and add the code to your Custom Code Box (Analytics). If you add your custom text here, it will be displayed on member profile pages directly above the Status Box. You need to keep this text simple. Be sure you do not use single quote marks within your message. If you want, you can add simple links like this: <a href="http://jensocial.com">Visit JenSocial</a> This is a great way to make announcements to your members.<br>Here is the code for the JenSocial Banner, and it is clickable.<center><a title="JenSocial Network Directory - Free Directory for All Social Networks" href="http://jensocial.com" target="_blank"><img border=1 src="http://storage.ning.com/topology/rest/1.0/file/get/1061451822?profile=original" alt="JenSocial.com: Social Network Directory and Network Creator Hub"></a></center></div>');
});
</script>

ISSUE REPORTED ON AUGUST 31, 2010
Everyone,
Please make sure your Status Box is editable. Evelyn has reported a serious problem. I will look into this in detail tomorrow evening, out of office tomorrow except via mobile.

If you cannot edit your Status Box, please see TJ's alternate solution:
http://creators.ning.com/xn/detail/4244211:Comment:286626

---

Add Text Box Below Sign-In/Hello Box, on Member Profile Pages:
Example Code Inserting Text Box Below Sign-In/Hello Box:

<!-- Add Message to Profile Pages -->
<script type="text/javascript">
x$(document).ready(function() {
x$(".xg_widget_profiles_profile_show #xg_module_account").after('<div style="font-size:12px; color:#000000; background-color:#FFFFFF; margin-top:15px; margin-bottom:15px; border:1px #333333 solid; padding:5px;" class="xg_module">To add a custom message box on member profile pages, change this text, and add the code to your Custom Code Box (Analytics). If you add your custom text here, it will be displayed on member profile pages directly below the Sign-In/Hello Box. You need to keep this text simple. Be sure you do not use single quote marks within your message. If you want, you can add simple links like this: <a href="http://jensocial.com">Visit JenSocial</a> This is a great way to make announcements to your members.</div>');
});
</script>

ISSUE REPORTED ON AUGUST 31, 2010
Everyone,
Please make sure your Status Box is editable, if you used the first text box example. A member on Creators (Evelyn) has reported a
serious problem. I will look into this in detail tomorrow evening, out
of office tomorrow except via mobile.

If you cannot edit your Status Box, please see TJ's alternate solution:
http://creators.ning.com/xn/detail/4244211:Comment:286626

Thanks!
Jen