How do I set a condition to only show a block to members of a group?
Programmatically you can use Group::getMember() to determine if an account is a member of a group.
if ($group-&gt;getMember(\Drupal::currentUser())) {
  // the current user is a member of the group.
}

Note that Group::getMember() returns a GroupMembership entity.
You could then create a condition plugin, using the above methodology to determine if the current user is a member of the group. I've created a tutorial on how to use the Conditions API to create conditions plugins, using this example of a condition where a member is a member of the group: https://www.jaypan.com/tutorial/custom-block-visibility-plugins-and-condition-plugin-api