Team fighting for Rell is like many other tank Supports. She may wish to go in to look for an aggressive play with her E and then use her Ultimate to pull the enemies towards her team. Just like all other tank Supports, she will want to go in and then fall back and peel for her carries. It’s not advised to pull incredibly strong enemies like Darius or Renekton into her ADC if the enemy is fed.
Her Ultimate is very versatile. You can use it as an aggressive tool to pick players off, or you can use it as a utility tool to peel for your ADC and keep the enemy frontline away from them. It is a very diverse tool which can be used in a plethora of ways.
Try out different team fighting techniques when playing Rell to see which suits your and her playstyle.