diff --git a/database.sql b/database.sql
index 4237bcce34..ea129109dc 100644
--- a/database.sql
+++ b/database.sql
@@ -644,10 +644,13 @@ CREATE TABLE IF NOT EXISTS `group` (
 	`uid` mediumint unsigned NOT NULL DEFAULT 0 COMMENT 'Owner User id',
 	`visible` boolean NOT NULL DEFAULT '0' COMMENT '1 indicates the member list is not private',
 	`deleted` boolean NOT NULL DEFAULT '0' COMMENT '1 indicates the group has been deleted',
+	`cid` int unsigned COMMENT 'Contact id of forum. When this field is filled then the members are synced automatically.',
 	`name` varchar(255) NOT NULL DEFAULT '' COMMENT 'human readable name of group',
 	 PRIMARY KEY(`id`),
 	 INDEX `uid` (`uid`),
-	FOREIGN KEY (`uid`) REFERENCES `user` (`uid`) ON UPDATE RESTRICT ON DELETE CASCADE
+	 INDEX `cid` (`cid`),
+	FOREIGN KEY (`uid`) REFERENCES `user` (`uid`) ON UPDATE RESTRICT ON DELETE CASCADE,
+	FOREIGN KEY (`cid`) REFERENCES `contact` (`id`) ON UPDATE RESTRICT ON DELETE CASCADE
 ) DEFAULT COLLATE utf8mb4_general_ci COMMENT='privacy groups, group info';
 
 --
diff --git a/doc/database/db_group.md b/doc/database/db_group.md
index 1892de3e40..ad7fa4a3dd 100644
--- a/doc/database/db_group.md
+++ b/doc/database/db_group.md
@@ -6,13 +6,14 @@ privacy groups, group info
 Fields
 ------
 
-| Field   | Description                                | Type               | Null | Key | Default | Extra          |
-| ------- | ------------------------------------------ | ------------------ | ---- | --- | ------- | -------------- |
-| id      | sequential ID                              | int unsigned       | NO   | PRI | NULL    | auto_increment |
-| uid     | Owner User id                              | mediumint unsigned | NO   |     | 0       |                |
-| visible | 1 indicates the member list is not private | boolean            | NO   |     | 0       |                |
-| deleted | 1 indicates the group has been deleted     | boolean            | NO   |     | 0       |                |
-| name    | human readable name of group               | varchar(255)       | NO   |     |         |                |
+| Field   | Description                                                                               | Type               | Null | Key | Default | Extra          |
+| ------- | ----------------------------------------------------------------------------------------- | ------------------ | ---- | --- | ------- | -------------- |
+| id      | sequential ID                                                                             | int unsigned       | NO   | PRI | NULL    | auto_increment |
+| uid     | Owner User id                                                                             | mediumint unsigned | NO   |     | 0       |                |
+| visible | 1 indicates the member list is not private                                                | boolean            | NO   |     | 0       |                |
+| deleted | 1 indicates the group has been deleted                                                    | boolean            | NO   |     | 0       |                |
+| cid     | Contact id of forum. When this field is filled then the members are synced automatically. | int unsigned       | YES  |     | NULL    |                |
+| name    | human readable name of group                                                              | varchar(255)       | NO   |     |         |                |
 
 Indexes
 ------------
@@ -21,6 +22,7 @@ Indexes
 | ------- | ------ |
 | PRIMARY | id     |
 | uid     | uid    |
+| cid     | cid    |
 
 Foreign Keys
 ------------
@@ -28,5 +30,6 @@ Foreign Keys
 | Field | Target Table | Target Field |
 |-------|--------------|--------------|
 | uid | [user](help/database/db_user) | uid |
+| cid | [contact](help/database/db_contact) | id |
 
 Return to [database documentation](help/database)
diff --git a/static/dbstructure.config.php b/static/dbstructure.config.php
index 014185c92a..0cf8543718 100644
--- a/static/dbstructure.config.php
+++ b/static/dbstructure.config.php
@@ -704,11 +704,13 @@ return [
 			"uid" => ["type" => "mediumint unsigned", "not null" => "1", "default" => "0", "foreign" => ["user" => "uid"], "comment" => "Owner User id"],
 			"visible" => ["type" => "boolean", "not null" => "1", "default" => "0", "comment" => "1 indicates the member list is not private"],
 			"deleted" => ["type" => "boolean", "not null" => "1", "default" => "0", "comment" => "1 indicates the group has been deleted"],
+			"cid" => ["type" => "int unsigned", "foreign" => ["contact" => "id"], "comment" => "Contact id of forum. When this field is filled then the members are synced automatically."],
 			"name" => ["type" => "varchar(255)", "not null" => "1", "default" => "", "comment" => "human readable name of group"],
 		],
 		"indexes" => [
 			"PRIMARY" => ["id"],
 			"uid" => ["uid"],
+			"cid" => ["cid"],
 		]
 	],
 	"group_member" => [