Bulk assign of content to groups

I'd be interested in that, too.
To me it is a critical question in order to find out how to migrate an existing site to the intriguing idea of the group module.

Well, you need a GroupContent entity for every Node belonging to a Group, so my advice would be to use the API that's available: GroupInterface::addContent()

The steps to take are roughly:

1. Make sure you have a GroupType
2. Make sure the GroupType has the right plugins enabled (GroupType::installContentPlugin(); read deprecation notice)
3. Create a Group of said GroupType
4. Start adding the nodes using $group->addContent($node, 'group_node:NODE_TYPE_HERE');

Either that, or you set up your migration in three steps:

1. Make sure a group exists (see above)
2. Migrate nodes first
3. Manually create GroupContent entities to avoid having to use the API

But that would require more thorough knowledge of the Group module's internals.

is there a relationship via dynamic entityrelationship or some other module that we can use?

Possible module: https://www.drupal.org/project/group_action