When you want to disable welcome messages from being sent to new group members who are added to a group by role (e.g., a created role called “Readers” that only has Blogs – Read and Forums – Read permissions), you can do this using the $core_v2_group extension in the group_roleadded.xml email template.
Add the following logic to the template’s subject:
#if($core_v2_group.Current.Id == <Group_Id_to_ignore>)
CancelEmail() will prevent the email from being sent. You can add that logic for as many groups as desired.
The same logic can be applied for when individual users are added to a group, just update the group_useradded.xml template instead.