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Entity Group Tool

Not a game mode, but a helper utility for placing entity groups in your own mutations)
Read the full docs here
http://www.leeland.net/entitygroupspawner.html

Launching
The default method, from the console, map mapname groupspawner

This will load the default entity groups (groupspawn_defaults.nut) and custom entity groups (groupspawn_user.nut),

if it exists. Some maps will crash due to EDICT limits, if that's the case, you can load the lite method which ONLY loads your custom file (groupspawn_user.nut), of course you need to create it first.

from the console, map mapname groupspawner_lite

2013.09.05 - Added !run chat command. Will save ents, reload map with the mutation you're editing
2013.09.05 - UPDATE - in Beta version, the entities files are now saved under ems/maps, and the game will load the entity file from that location (if not found under maps) allowing you to quickly switch back and forth between the tool and your own mutation without cutting and pasting the file to maps.