Hi,
You are on the right track by using $core_v2_activityMessage.List(). Under Widget Studio in the API Documentation that extension has an overload method. Here is a sample below that may help:
#set($user = $core_v2_user.Accessing)
#set($group = $core_v2_group.Current)
#set($includeSubGroups = true)
#set($activityList = $core_v2_activityMessage.List(
"%{ GroupId = $group.Id, IncludeSubGroups = $includeSubGroups, UserId = $user.Id}"))
#foreach($activity in $activityList)
<div>$activity.Title</div>
<div>$activity.Body</div>
#end
It is strongly suggested to also specify a BeforeDate when including sub-groups to limit the number of messages in the response. You may also want to add paging and other elements.
Thanks.