The feature will be available if it is enabled by the Workspace admin under Manage Workspace. Settings related to privacy and security will not change.
TABLE OF CONTENTS
How does @mention work
Where does @mention work
Notifications
How to edit the username
On successful login to a Workspace by a new member, a screen to set profile/username pops up as shown in the image below.
The username should be between 6 and 40 characters and use only lowercase letters (a-z), numbers, and periods. It can be a combination of letters and numbers.
Username creation and @mentions will be allowed in the communities only if the Workspace admin had enabled the setting Username setup required under Manage Workspace >> Customization >> Customize Site Behavior >> Miscellaneous
How does @mention work
@mentions will function in line with commonly accepted Internet standards. A user typing @“username” will see an auto-complete list of matching usernames to select from. The selected user will receive an email pointing toward the referenced content. In private campaigns, it is not possible to send @mention notifications to members who do not have access to the campaign. This feature also has curse filtering applicable in case an inappropriate name is used as a user name.
Where does @mention work
A member can use @mentions while commenting on an idea that looks like the image below.
It also can be used to mention while submitting an idea in the description field. 
The @mention feature is also available to members who have access to the Stage-specific comments tab. For example: Build Stage, Review Stage, Refine Stage, Assessment Stage.
The feature is also available on the Refine stage answer fields.
Notifications
A notification email is sent to the member on being mentioned in the community. The member will receive the email notification only if Receive mention email setting is enabled from Profile >> Settings >> Notification settings.
Apart from this, an in-app notification is also received by the member being mentioned at 2 places:
1. Topbar Notification
2. Profile page >> Notifications >> Mentions
How to Edit the Username
The member can always edit or change their username from their Profile >> Settings >>Profile information >> Username if it is a non-SSO community.
The member of an SSO community will be able to edit or change the username from the Profile only if the Workspace admin has enabled 'Allow members/persons to change username' setting is enabled in Manage Workspace >> Security >> Authentication >> Single Signon under the Settings icon. If this setting is not enabled the username field will be greyed out in the Personal settings.
If the setting Enable Username Prompt is disabled it will never ask the new member to create a username. The system will automatically generate a username if not provided by the IDP or if the provided username is not valid. If the member has a valid username used earlier and the one provided by the IDP is invalid it will retain the old valid username.
SSO communities can pass usernames via ADFS as a SAML attribute and map it to the username. In case of SSO communities, the members will be asked for a username only if the setting Enable Username Prompt in the SSO General Settings is enabled.