Index: binaries/data/config/default.cfg
===================================================================
--- binaries/data/config/default.cfg
+++ binaries/data/config/default.cfg
@@ -373,6 +373,7 @@
 rankabovestatusbar = true         ; Show rank icons above status bars
 experiencestatusbar = true        ; Show an experience status bar above each selected unit
 respoptooltipsort = 0             ; Sorting players in the resources and population tooltip by value (0 - no sort, -1 - ascending, 1 - descending)
+disjointcontrolgroups = true      ; Whether entities are allowed to be in multiple control groups.
 
 [gui.session.minimap]
 blinkduration = 1.7               ; The blink duration while pinging
Index: binaries/data/mods/public/gui/options/options.json
===================================================================
--- binaries/data/mods/public/gui/options/options.json
+++ binaries/data/mods/public/gui/options/options.json
@@ -540,6 +540,12 @@
 				"tooltip": "Color of enemies when diplomacy colors are enabled.",
 				"config": "gui.session.diplomacycolors.enemy",
 				"callback": "updateDisplayedPlayerColors"
+			},
+			{
+				"type": "boolean",
+				"label": "Allow units only in single control group",
+				"tooltip": "If enabled, remove entities from a previous control group when adding to another control group. Otherwise allow entities to be in multiple control groups simultaneously.",
+				"config": "gui.session.disjointcontrolgroups"
 			}
 		]
 	}
Index: binaries/data/mods/public/gui/session/selection.js
===================================================================
--- binaries/data/mods/public/gui/session/selection.js
+++ binaries/data/mods/public/gui/session/selection.js
@@ -457,12 +457,18 @@
 		this.groups[i] = new EntityGroups();
 }
 
+/**
+ * Add entities to a group.
+ * @param {string} groupName - The number of the group to add the entities to.
+ * @param {number[]} ents - The entities to add to the group.
+ */
 EntityGroupsContainer.prototype.addEntities = function(groupName, ents)
 {
-	for (let ent of ents)
-		for (let group of this.groups)
-			if (ent in group.ents)
-				group.removeEnt(ent);
+	if (Engine.ConfigDB_GetValue("user", "gui.session.disjointcontrolgroups") == "true")
+		for (let ent of ents)
+			for (let group of this.groups)
+				if (ent in group.ents)
+					group.removeEnt(ent);
 
 	this.groups[groupName].add(ents);
 };