Latest NX Continuous Release updates (since1899) made available Design Groups.
https://docs.plm.automation.siemens.com/tdoc/nx/1899/nx_help#uid:index_whatsnew:id1241822:xid1749001:xid1749006:xid1753909
However the NX on-line documentation does not mention any license requirement.
Solution
Actually these NX Design Groups check out a User Defined Features license, hence you will observe several check IN/OUT in your Flexlm log file.
17:13:48 (ugslmd) OUT: "usr_defined_features" user1@PC12
17:13:49 (ugslmd) IN: "usr_defined_features" user1@PC12
...
17:24:48 (ugslmd) OUT: "usr_defined_features" user1@PC12
17:24:48 (ugslmd) IN: "usr_defined_features" user1@PC12
Currently (in NX 1899) there are still some inconsistencies, that's why you will able to create Design Groups without checking out that license feature but in any case the intended behavior is that license will be checked out (and almost immediately in) when creating, reorganizing and moving features to Design Groups.
Hardware/Software Configuration
Platform: na
OS: all
OS Version: n/a
Product: NX
Application: DESIGN
Version: V1899
Function: DESIGN_GROUPS
Ref: 002-8016757