I'm not sure we've had a word for missing someone. You could perhaps use the verb zigerelat which means to need, or to require. Anha zigerek yeroon would mean "I need you".
For farwell or simply goodbye there are several options used.
Fonas chek is a simply good bye and literally means "hunt well"
Dothras chek can be used in a similar way in parting, it means "ride well"
Hajas is also used, it means "Be strong"
A little more elaborate would be Shieraki gori ha yeraan which means "The stars are charging for you". This is said as a good luck or well-wishing of someone who is going into battle.