Edit Email Template to Add Additional Parameter for Email Campaign Tracking

This customization to the URL would require a custom token:  Registering Tokens for Templates 

You would use this token in place of the Group: Url token in the template.

Thanks Steven,

You can use the same data type Id to register more tokens for an existing type. For instance, supplying Apis.Get<IGroups>().ContainerTypeId as the dataTypeId would render your custom token along with the other group tokens.

Thanks Steven.

tokenController.Register((ITokenizedTemplateLinkUrlToken) new TokenizedTemplateLinkUrlToken("Campaign Group url","Campaign Group url",this._controller,
Guid.Parse("DC9CAAFD-89B9-4EE9-B5C7-403AB6FE1BF9"),new Guid?(Apis.Get<IGroups>().ContainerTypeId) ,
(Func<TemplateContext, string>) (context => (this.Group(context).Url) + "?utm_source=Weekly%20Digest&utm_medium=Email&utm_campaign=" + this.Group(context).Name )));

Is this right?, can you help me in implementing this token? Can we add the above code in existing GroupTokens.cs file?

You'll need to create your own plugin and add the compiled dll to your Community installation. More details here:  Setting Up Your Development Environment 

I'd also recommend using the IUrl API method Adjust to attach the query string to the url in a robust manner:  Url In-Process API Service