I can't be added to already existed kobo form

Hi @Gharam_medhat, kindly please be informed that KoBoToolbox has two public servers viz. the Global and the EU.

Please be informed that sharing of projects is only possible within the same server, i.e. either within the Global server or within the EU server. It’s impossible to share a survey project between Global to EU or vice versa.

You could also have a look at our support article MANAGING PERMISSIONS to learn more about sharing projects with other users.