I am unable to change group membership, despite having globalgroupmembership should allow me to add/remove anyone from any global group.
Special:GlobalUserRights/Martin_Urbanec at meta looks like this:
I can demote myself from steward, or promote someone else to steward, but not add other global groups.
Since this blocks steward's work significantly, I am boldly triaging this as a train blocker. I have verified this was caused in wmf.34 by temporarily adding globalgrouppermissions to the global steward group, and loading the page at cswiki.