Is it possible to link or mention group in the Yammer message?
For example you can mention people using @john and then select from the list the person. Does something similar exist for groups and if not what is the best way to link to the group?
No, the equivalent functionality is not available for groups. The best way to link to a group is to use the "web URL" for the group. This is a URL of the form:
https://www.yammer.com/{network permalink}/#/threads/inGroup?type=in_group&feedId={group id}
You can also link to a group using a URL of this form:
https://www.yammer.com/{network permalink}/groups/{group permalink}
Unfortunately, this URL can change over time if the group is ever renamed. The web URL, group ID, network permalink, and group permalink are all available via the API. Alternatively, the web URL can be obtained by copying the URL behind any group link.