There 2 types of groups with this plugin.

first the default GROUP from rust
and second the clans 

even if i set clan max 3 and group max 3 they can snitch this system by doing this:

create clan go to max 3 member

now leave group !( not the clan )

invite member to a new group

wola you get 5 member

is here a way to prevent this?

Hi.

You can handle the size of default groups from rust by this command in console:

relationshipmanager.maxteamsize 3

normaly there is max size 8.

Then max Size in Clan-Config = 3.
If you write down this command in serverauto.cfg this should stay active after restart.

ive done this already and its not a solution.
you can have 3 clan member AND 3 groups member