How can one invite a person to prayer when he knows it is obligatory but sometimes abandons it, and what are the appropriate phrases for encouragement and deterrence in that regard?

One can remind the one who abandons prayer and advise him by clarifying the ruling on one who abandons it: he is an infidel, outside the fold of Islam; none of his deeds are accepted. His marriage contract must be annulled, his slaughtered animal is not to be eaten, and he is not to be washed, prayed over, or buried in Muslim graveyards. Furthermore, the one who abandons the prescribed prayer will be tormented in his grave. It has been authentically reported from the Prophet (peace be upon him) and the people of knowledge that intentionally abandoning prayer without a valid excuse until its time has passed renders one an infidel.

As for congregational prayer in the mosque, it is a sign distinguishing the believer from the hypocrite. The Companions considered one who failed to attend it a hypocrite. A Muslim must maintain performing it at its prescribed times with the Muslim congregation. It is not permissible to delay prayer beyond its time or combine prayers for an entire day except for a legitimate excuse. The righteous predecessors were very keen on congregational prayer, to the extent that some of them would weep if they missed the first takbeer or the congregational prayer, and they would attend it even in the severest states of illness.