1) When I am praying my optional prayer after the obligatory Jama'a prayer - Thuhr, someone comes and joins me, so what shall I do?
2) While on prayer - the Imaam left suddenly, so what shall the Ma'moom do, standing behind Imaam while on mass obligatory prayer. Is it allowed for an Imaam - to leave out from the prayer with reasons?
3) Entering to Masjid - is the two Raka'h prayers are restricted in any time, as some are not taking this Sunnah in the time of Maghrib? Your kind answer at the earliest is highly appreciated.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

With regard to the first and third question, please refer to Fataawa 86581 and 82719.

As regards what the people led in prayer should do if the Imaam abandons the prayer, the answer is that if the Imaam has any excuse for not continuing the prayer, like having his ablution nullified during the prayer and the like, or he remembers that his clothes are touched or stained by an impurity, then in such a case, it is permissible for him to appoint someone who started the prayer at the same time with him before his ablution was invalidated. In case the Imaam did not appoint someone to lead the congregation in prayer, it is permissible for the congregation to put someone forward to lead and continue the prayer. If no one is appointed to lead the prayer, there is no harm for anyone to move forward without being directed by the congregation to lead the prayer. In case there was no one at all who moved forward to lead the congregation in prayer, then each individual should continue his prayer individually, and this does not invalidate their prayer.

Allaah Knows best.