I am in a group which has Full Permission (site collection owners).
However i cant add users to groups that i am not a member off.
I thought this would give me full overriding permissions.
Am i misunderstanding something?
In group settings there is a page "Group Settings" which can be accessed thorough: (Group Page) > Settings > Group Settings
3rd section is group settings section where you can set "Who can edit the membership of the group?" which in default is "Group Owner". In the same page you can change group owner which in default is 1st site collection administrator.
In your case I believe that in all groups, "Who can edit the membership of the group?" is set to "Group Members" and you are not the group owner in any of the groups. Therefore, you are just able to edit membership in those groups you are a member of. Other group memberships can only be edited by site collection administrator.