diff --git a/src/api/groups.js b/src/api/groups.js
index 3f251d189c20..41c8b089b9b4 100644
--- a/src/api/groups.js
+++ b/src/api/groups.js
@@ -3,6 +3,8 @@
 const privileges = require('../privileges');
 const events = require('../events');
 const groups = require('../groups');
+const user = require('../user');
+const meta = require('../meta');
 
 const groupsAPI = module.exports;
 
@@ -27,9 +29,60 @@ groupsAPI.create = async function (caller, data) {
 	return groupData;
 };
 
-// groupsAPI.join = async function (caller, data) {
-// 	// TODO:
-// };
+groupsAPI.join = async function (caller, data) {
+	if (caller.uid <= 0 || !data.uid) {
+		throw new Error('[[error:invalid-uid]]');
+	}
+
+	const isSelf = parseInt(caller.uid, 10) === parseInt(data.uid, 10);
+	const groupName = await groups.getGroupNameByGroupSlug(data.slug);
+	if (!groupName) {
+		throw new Error('[[error:no-group]]');
+	}
+
+	if (groups.systemGroups.includes(groupName) || groups.isPrivilegeGroup(groupName)) {
+		throw new Error('[[error:not-allowed]]');
+	}
+
+	const [groupData, isCallerAdmin, isCallerOwner, userExists] = await Promise.all([
+		groups.getGroupData(groupName),
+		user.isAdministrator(caller.uid),
+		groups.ownership.isOwner(caller.uid, groupName),
+		user.exists(data.uid),
+	]);
+
+	if (!userExists) {
+		throw new Error('[[error:invalid-uid]]');
+	}
+
+	if (!meta.config.allowPrivateGroups && isSelf) {
+		// all groups are public!
+		await groups.join(groupName, data.uid);
+		logGroupEvent(caller, 'group-join', {
+			groupName: groupName,
+			targetUid: data.uid,
+		});
+		return;
+	}
+
+	if (groupData.private && groupData.disableJoinRequests) {
+		throw new Error('[[error:group-join-disabled]]');
+	}
+
+	if ((!groupData.private && isSelf) || isCallerAdmin || isCallerOwner) {
+		await groups.join(groupName, data.uid);
+		logGroupEvent(caller, 'group-join', {
+			groupName: groupName,
+			targetUid: data.uid,
+		});
+	} else if (isSelf) {
+		await groups.requestMembership(groupName, caller.uid);
+		logGroupEvent(caller, 'group-request-membership', {
+			groupName: groupName,
+			targetUid: data.uid,
+		});
+	}
+};
 
 // groupsAPI.leave = async function (caller, data) {
 // 	// TODO:
diff --git a/src/controllers/write/groups.js b/src/controllers/write/groups.js
index 3b887122c133..d6c8b041d4a0 100644
--- a/src/controllers/write/groups.js
+++ b/src/controllers/write/groups.js
@@ -5,7 +5,6 @@ const validator = require('validator');
 const user = require('../../user');
 const groups = require('../../groups');
 const events = require('../../events');
-const meta = require('../../meta');
 const slugify = require('../../slugify');
 const notifications = require('../../notifications');
 const api = require('../../api');
@@ -40,40 +39,8 @@ Groups.delete = async (req, res) => {
 };
 
 Groups.join = async (req, res) => {
-	const group = await groups.getByGroupslug(req.params.slug, {
-		uid: req.params.uid,
-	});
-	const [isCallerOwner, userExists] = await Promise.all([
-		groups.ownership.isOwner(req.user.uid, group.name),
-		user.exists(req.params.uid),
-	]);
-
-	if (!userExists) {
-		throw new Error('[[error:invalid-uid]]');
-	} else if (group.isMember) {
-		// No change
-		return helpers.formatApiResponse(200, res);
-	}
-
-	if (!res.locals.privileges.isAdmin) {
-		// Admin and privilege groups unjoinable client-side
-		if (groups.systemGroups.includes(group.name) || groups.isPrivilegeGroup(group.name)) {
-			throw new Error('[[error:not-allowed]]');
-		}
-
-		if (!isCallerOwner && parseInt(meta.config.allowPrivateGroups, 10) !== 0 && group.private) {
-			await groups.requestMembership(group.name, req.params.uid);
-		} else {
-			await groups.join(group.name, req.params.uid);
-		}
-	} else {
-		await groups.join(group.name, req.params.uid);
-	}
-
+	await api.groups.join(req, req.params);
 	helpers.formatApiResponse(200, res);
-	logGroupEvent(req, 'group-join', {
-		groupName: group.name,
-	});
 };
 
 Groups.leave = async (req, res) => {
diff --git a/src/socket.io/groups.js b/src/socket.io/groups.js
index d7e72ede6aaf..cacc69b83cf5 100644
--- a/src/socket.io/groups.js
+++ b/src/socket.io/groups.js
@@ -2,7 +2,6 @@
 
 const validator = require('validator');
 const groups = require('../groups');
-const meta = require('../meta');
 const user = require('../user');
 const utils = require('../utils');
 const slugify = require('../slugify');
@@ -21,52 +20,8 @@ SocketGroups.before = async (socket, method, data) => {
 
 SocketGroups.join = async (socket, data) => {
 	sockets.warnDeprecated(socket, 'PUT /api/v3/groups/:slug/membership/:uid');
-
-	if (socket.uid <= 0) {
-		throw new Error('[[error:invalid-uid]]');
-	}
-
-	if (typeof data.groupName !== 'string') {
-		throw new Error('[[error:invalid-group-name]]');
-	}
-
-	if (groups.systemGroups.includes(data.groupName) || groups.isPrivilegeGroup(data.groupName)) {
-		throw new Error('[[error:not-allowed]]');
-	}
-
-	const exists = await groups.exists(data.groupName);
-	if (!exists) {
-		throw new Error('[[error:no-group]]');
-	}
-
-	if (!meta.config.allowPrivateGroups) {
-		await groups.join(data.groupName, socket.uid);
-		logGroupEvent(socket, 'group-join', {
-			groupName: data.groupName,
-		});
-		return;
-	}
-
-	const results = await utils.promiseParallel({
-		isAdmin: await user.isAdministrator(socket.uid),
-		groupData: await groups.getGroupData(data.groupName),
-	});
-
-	if (results.groupData.private && results.groupData.disableJoinRequests) {
-		throw new Error('[[error:group-join-disabled]]');
-	}
-
-	if (!results.groupData.private || results.isAdmin) {
-		await groups.join(data.groupName, socket.uid);
-		logGroupEvent(socket, 'group-join', {
-			groupName: data.groupName,
-		});
-	} else {
-		await groups.requestMembership(data.groupName, socket.uid);
-		logGroupEvent(socket, 'group-request-membership', {
-			groupName: data.groupName,
-		});
-	}
+	const slug = await groups.getGroupField(data.groupName, 'slug');
+	await api.groups.join(socket, { slug: slug, uid: data.uid || socket.uid });
 };
 
 SocketGroups.leave = async (socket, data) => {
diff --git a/test/groups.js b/test/groups.js
index a388a5a929a4..1605c712e8bd 100644
--- a/test/groups.js
+++ b/test/groups.js
@@ -640,7 +640,7 @@ describe('Groups', function () {
 				const isMember = await Groups.isMember(newUid, 'administrators');
 				assert(!isMember);
 			} catch (err) {
-				assert.strictEqual(err.message, '[[error:invalid-group-name]]');
+				assert.strictEqual(err.message, '[[error:no-group]]');
 			}
 			meta.config.allowPrivateGroups = oldValue;
 		});