I'm currently running a server with clans of 5 and have rust io clans installed but is there an extension available that forces people to go in a clan, so disable making a in-game team without having a clan 

have you tried relationshipmanager.maxteamsize 0 ?

Serray

have you tried relationshipmanager.maxteamsize 0 ?

No? What does that do exactly?

Well, this command sets the maximum teamsize possible. With 0 or 1 it shouldnt be able to invite someone else into a team

Is there a way to add a message alongside it?

Any outcome of this?

Yoooo. just use ClansReborn (not gonna link the website), use brain and google to find this best plugin... It's the same as clans from uMod but like plus version. You can make clans reborn to handle rust teams and tweak other settings to achieve the final result:

Clicking the Create Team vanilla button won't make a team and will send in a chat message to use /clan create Tag "descr" then after creating the clan will put you in a rust team... Then everyone else you want to play with either has to be invited via /clan invite NAME or simply press Invite on his character like you do in vanilla rust team system.

Yup I already moved on with clans REBORN 😁