This is a very simple plugin that will create a new mask image stack in which every point within a certain distance of points over some threshold in the original image are set. It might be simpler to see it as a way of "growing" an existing mask by a certain specified amount, as shown in the image at the top right.
Please let me know of any problems you have with this plugin or any suggestions you have. I hope it is of some use to you.
This plugin was written by Mark Longair.
This plugin is now bundled as part of Fiji (a distribution of ImageJ with many useful plugins), and no longer supported as a standalone ImageJ plugin. Please download and install Fiji and you will find the plugin in "Plugins > Process > Mask of Nearby Points"
The initial dialog box looks like this:
... and should be self-explanatory.
This plugin is licensed under the GPL (the GNU General Public License version 3). The source code for the plugin is available from the Fiji git repository.