diff --git a/src/groups/invite.ts b/src/groups/invite.ts
new file mode 100644
index 0000000..c8bb2e1
--- /dev/null
+++ b/src/groups/invite.ts
@@ -0,0 +1,120 @@
+'use strict';
+
+const _ = require('lodash');
+
+const db = require('../database');
+const user = require('../user');
+const slugify = require('../slugify');
+const plugins = require('../plugins');
+const notifications = require('../notifications');
+
+module.exports = function (Groups) {
+	Groups.getPending = async function (groupName: string) {
+		return await Groups.getUsersFromSet(`group:${groupName}:pending`, ['username', 'userslug', 'picture']);
+	};
+
+	Groups.getInvites = async function (groupName: string) {
+		return await Groups.getUsersFromSet(`group:${groupName}:invited`, ['username', 'userslug', 'picture']);
+	};
+
+	Groups.requestMembership = async function (groupName: string, uid: string) {
+		await inviteOrRequestMembership(groupName, uid, 'request');
+		const { displayname } = await user.getUserFields(uid, ['username']);
+
+		const [notification, owners] = await Promise.all([
+			notifications.create({
+				type: 'group-request-membership',
+				bodyShort: `[[groups:request.notification-title, ${displayname}]]`,
+				bodyLong: `[[groups:request.notification-text, ${displayname}, ${groupName}]]`,
+				nid: `group:${groupName}:uid:${uid}:request`,
+				path: `/groups/${slugify(groupName)}`,
+				from: uid,
+			}),
+			Groups.getOwners(groupName),
+		]);
+
+		await notifications.push(notification, owners);
+	};
+
+	Groups.acceptMembership = async function (groupName:string , uid:string) {
+		await db.setsRemove([`group:${groupName}:pending`, `group:${groupName}:invited`], uid);
+		await Groups.join(groupName, uid);
+
+		const notification = await notifications.create({
+			type: 'group-invite',
+			bodyShort: `[[groups:membership.accept.notification-title, ${groupName}]]`,
+			nid: `group:${groupName}:uid:${uid}:invite-accepted`,
+			path: `/groups/${slugify(groupName)}`,
+			icon: 'fa-users',
+		});
+		await notifications.push(notification, [uid]);
+	};
+
+	Groups.rejectMembership = async function (groupNames: string |string[], uid: string) {
+		if (!Array.isArray(groupNames)) {
+			groupNames = [groupNames];
+		}
+		const sets: string[] = [];
+		groupNames.forEach(groupName => sets.push(`group:${groupName}:pending`, `group:${groupName}:invited`));
+		await db.setsRemove(sets, uid);
+	};
+
+	Groups.invite = async function (groupName: string, uids: string | string[]) {
+		uids = Array.isArray(uids) ? uids : [uids];
+		uids = await inviteOrRequestMembership(groupName, uids, 'invite');
+
+		const notificationData = await Promise.all(uids.map(uid => notifications.create({
+			type: 'group-invite',
+			bodyShort: `[[groups:invited.notification-title, ${groupName}]]`,
+			bodyLong: '',
+			nid: `group:${groupName}:uid:${uid}:invite`,
+			path: `/groups/${slugify(groupName)}`,
+			icon: 'fa-users',
+		})));
+
+		await Promise.all(uids.map((uid, index) => notifications.push(notificationData[index], uid)));
+	};
+
+	async function inviteOrRequestMembership(groupName: string, uids: string | string[], type: string) {
+		uids = Array.isArray(uids) ? uids : [uids];
+		uids = uids.filter(uid => parseInt(uid, 10) > 0);
+		const [exists, isMember, isPending, isInvited] = await Promise.all([
+			Groups.exists(groupName),
+			Groups.isMembers(uids, groupName),
+			Groups.isPending(uids, groupName),
+			Groups.isInvited(uids, groupName),
+		]);
+
+		if (!exists) {
+			throw new Error('[[error:no-group]]');
+		}
+
+		uids = uids.filter((uid, i) => !isMember[i] && ((type === 'invite' && !isInvited[i]) || (type === 'request' && !isPending[i])));
+
+		const set = type === 'invite' ? `group:${groupName}:invited` : `group:${groupName}:pending`;
+		await db.setAdd(set, uids);
+		const hookName = type === 'invite' ? 'inviteMember' : 'requestMembership';
+		plugins.hooks.fire(`action:group.${hookName}`, {
+			groupName: groupName,
+			uids: uids,
+		});
+		return uids;
+	}
+
+	Groups.isInvited = async function (uids: string[], groupName: string) {
+		return await checkInvitePending(uids, `group:${groupName}:invited`);
+	};
+
+	Groups.isPending = async function (uids: string[], groupName: string) {
+		return await checkInvitePending(uids, `group:${groupName}:pending`);
+	};
+
+	async function checkInvitePending(uids: string | string[], set: any) {
+		const isArray = Array.isArray(uids);
+		uids = isArray ? uids as string[] : [uids] as string[]; 
+		const checkUids = uids.filter((uid: string) => parseInt(uid, 10) > 0);
+		const isMembers = await db.isSetMembers(set, checkUids);
+		const map = _.zipObject(checkUids, isMembers);
+		return isArray ? uids.map(uid => !!map[uid]) : !!map[uids[0]];
+	}
+};