To join a WhatsApp group link by groups, just click, click on the link, then you will be asked to confirm if you want to join… if you are sure you want to join, tap or tap again..
However, you cannot become an administrator without the administrator’s permission. this would be contrary to the whole purpose of creating an administrator.
So, if you want to become an administrator. You either create your own group and invite people to join it, or ask the current administrator to name you as such..
I hope this was helpful..