Annika Backstrom

The “Who’s Online” sidebar widget shows a list of recently-active 
blog users. It has been built with Automattic’s <a href="http://p2theme.com/" rel="nofollow ugc">P2 
theme</a> in mind, but does not depend on P2 
functionality.
The most recently active users bubble up to the top. After one week, 
users fall off the bottom of the list and are hidden until they return. 
As of this writing the list does not support live sorting; the user 
order is fixed, and users missing from the list due to inactivity will 
be added to the top if they come online.

Works as expected

Who's reading your P2 blog right now? Keep track.

Daniel Bachhuber

Easily subscribe to a P2 comment thread without commenting using a “Follow” action link like WordPress.com has.
This plugin requires Jetpack and P2 to both be activated. It doesn’t yet allow you to unfollow, and there are some minor interface quirks.
Happy to receive feedback, questions, feature suggestions, and <a href="https://github.com/danielbachhuber/Jetpack-Follow-Link-for-P2" rel="nofollow ugc">pull requests</a>

Jetpack Follow Link for P2

Easily subscribe to a P2 comment thread without commenting using a "Follow" action link like WordPress.com has

Who's Online vs Jetpack Follow Link for P2