Hello raccoon, here's great web page to explain concept and purpose of Groups : https://help.eset.com/protect_admin/10.0/en-US/admin_groups.html?admin_groups.html
Basically,
Static groups are handled by user - you choose which computers goes to which static group (be aware 1 computer can be member of only one static group)
Dynamic groups are handled automatically by agent based on some conditions (e.g. Windows dynamic group can have only windows computers because they fulfil the condition (win computers will appear in dynamic group automatically). Linux or Mac computers wont be displayed there because they do not fill up condition)
For your case I would suggest to create Static group - e.g. "my_static_group_name" and move computers there by you .