It is not possible group chat with Firebase because client is not capable for listen multiple nodes at a time . Here i am attaching some other platforms that would be support whole features of popular chatting applications like whats app.
1 . Back end servers
-Openfire :- Its free and open source and good for developing applications like Telegram , But security is your own risk
-ejabberd :- It is a XMPP server also commercial use is payable, Whatsapp is running on this platform.
-Tigase :- It is another one for achieving the XMPP communication
Other client SDKs are there for creating XMPP client
-smack :- Its free and open source, but now its only for android platform, iOS pods will coming soon.
-Chat SDK :- Its free and open source but only for firebase , It is payable for XMPP version