All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
Merely not replying to their messages is not considered forbidden deserting. However, if they will gain benefit from your reply, then it is better to reply as much as you can so that you would earn the reward of this good deed (helping those in need of help). Allah, the Exalted, says (what means): {...and do good - that you may succeed.} [Quran 22:77] Moreover, the Prophet, sallallaahu ‘alayhi wa sallam, said, “Allah, the Exalted, helps His slave as long as the slave helps his brother.” [Muslim]
If you are busy and do not have time to help them, then you can ask them to give you more time in order to comfort them and to avoid giving the devil a chance to spoil your good relations with them.
Allah knows best.