See also the official documentation:

• For group members: Help Center > Members Manual > Understanding and using hashtags
• For group owners and moderators: Help Center > Group Owners Manual > Managing hashtags

Creating and Using Hashtags

A hashtag starts with a '#' and doesn't contain any spaces. It can be used to quickly locate/group related messages, manage topic duration, reply-to settings, and customize many other aspects of message delivery on a topic-by-topic basis.

There are a couple of ways to create a hashtag. By putting one in the subject line of a post, if allowed by the group setup (see Admin>Settings>Message Policies>Hashtag Permissions), it will automatically be created. Moderators with the proper permissions, as well as group owners, can create them in advance and also edit their properties.

In the message archive, and on your group home page, hashtags display as a colored rectangle with white letters. The color can be customized via the hashtag settings.

Creating Hashtags in Advance

1.  Go to your group

2.  Select the #HashTags button from the menu at left.

3.  On the screen that opens, click on the +Create HashTag button at the bottom of the page.

4.  Another screen will open where you can enter details, including Alias, Description, Duration, and Color.  There are also some options that you may want to set.

5.  When you're done, click on the blue +Create HashTag button at the bottom and you'll be taken back to the screen with the list of hashtags.

If you want to change settings for a hashtag, click on it's "edit" button to open an editor screen.  Be sure to click Update HashTag at the bottom when you're done making changes.  If you decide not to make a change, you can click on Cancel.  This is also where you can Delete the hashtag.

Retrofitting Hashtags 

To apply a hashtag to an existing collection of articles, you must view messages in topics view.

1. Narrow the list using search for key word

2. Check the box to collapse topics.  The list will now show the number of messages (inside a blue circle) in each topic displayed.

3. Read down the list and for any topic you wish to tag click on the blue circle to display that topic.
Note: I recommend that you hold down the ctl & shift key while you left-click on circle.  This will open it in a new tab.
This will preserve your Topics list for continued review.

4. At the top of the new tab you will see a button to "edit topic" - click this and

5a. If you have predefined the tags you wish to use then drop down the add tags and select from your predefined tags.

5b. If you need to define a new tag simply add a space and type it at the end of the topic line -- example #good-stuff

6. It is possible to assign multiple tags to a topic, up to 5.  When finished press update topic at the bottom of the page

7. If you remembered to open the topic in a new tab you can now close the active tab and you can continue the review
from where you left off.

Merging hashtags

If you subsequently find that you have accumulated several very similar hashtags and need to merge them, that can also be done. See https://groups.io/g/GroupManagersForum/topic/17911284 for details.

System hashtags

There are a few hashtag names used by the system as needed. Most are associated with notifications of some kind, but not always. Some examples of system hashtags:

#guidelines -- added to the corresponding Member Notice when it is sent out
#poll -- automatically added to any poll notifications
#chat -- automatically added to any chat notifications
#email -- automatically added to emails that come into your group via the email integration
#cal-invite, #cal-notice, #cal-reminder, #cal-summary, #cal-canceled -- added to the corresponding calendar event notifications
etc...

If these hashtags don't exist, they are created with default settings the first time a notification is generated that requires that hashtag. They can subsequently be edited and customized (with various delivery options and such), just like a hashtag you created manually, via the Hashtags menu item on the left-side menu bar. You can also proactively create them before they are used so they start out with the desired attributes.

At this time, there is nothing to keep someone for using these "system hashtags" for other purposes. This is the source of a frequent complaint in beta, where a subscriber could choose to append #chat or #poll to the subject of a message unrelated to an actual, ongoing chat or poll. 

Hashtag Placement and Limits

Hashtags are normally placed in the Subject line of a message. However, you can also place them in any field which will subsequently be used as a subject line for a notification. Examples of this are the Title field of a poll, or the Event Name of a calendar event.

The total number of hashtags in a subject line is currently limited to five (5). If you put in more than that, they will all appear in outgoing emails, but the extra ones will be stripped prior to saving it in the message archive.

Hashtag syntax

• starts with # (an octothorpe)
• may contain letters in upper and lower case spelling, example #Question, #question
  The system treats these as the same. If you already have a #Question hashtag it will not let you create one called #question.
  If a message comes in with #question in the subject line the tag will be altered to #Question before it posts.
• may contain hyphens - and/or underscores _, for example #Akita-in-Not, #EVAP_System