They spent the night outside Mina, and did not realise that Mina was close to

Praise be to Allah.

Firstly:

Staying overnight in Mina during the three days of at-tashreeq is one of the obligatory parts of pilgrimage, so whoever fails to stay there overnight with no excuse must offer a compensatory sacrifice according to the majority of scholars.

It says in al-Mawsoo‘ah al-Fiqhiyyah (17/58):

Staying overnight in Mina during the days of at-tashreeq is obligatory according to the majority of fuqaha’, and a compensatory sacrifice must be offered by the one who fails to do it with no excuse. End quote.

For more information, please see the answers to questions no. 21258 and 95374.

Secondly:

If a person fails to stay overnight in Mina when he is able to do so, but he is unaware of the boundaries of Mina, he is not regarded as being excused, because what he should do is ask about the location of Mina so that he can stay there overnight.

Shaykh Ibn Baaz (may Allah have mercy on him) said:

whoever fails to stay overnight in Mina due to ignorance of its boundaries when he is able to stay there overnight, must offer a compensatory sacrifice, because he has omitted an obligatory duty with no legitimate excuse, and what he should have done was to ask so that he could fulfil that obligatory duty.

End quote from Majmoo‘ Fataawa Ibn Baaz (16/149)

Based on that, if you had a place in Mina where you could stay overnight, then you have no excuse for omitting to do this obligatory duty, and for having failed to stay overnight on that night, you must offer charity, but not a compensatory sacrifice, because the sacrifice is only required for failing to spend all the nights in Mina.

An-Nawawi (may Allah have mercy on him) said:

If a person fails to spend all three nights of at-tashreeq in Mina, he must offer a compensatory sacrifice. … If he fails to stay there for two nights, then according to the more correct view, he must give two mudds of food in charity.

End quote from al-Majmoo‘ (8/225)

Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said:

From the words “if he does not stay there overnight,” it is understood that if he omitted one of the nights, then he does not have to offer a compensatory sacrifice, and that is the case; rather he has to feed one poor person if he omitted one night, and feed two poor persons if he omitted two nights. He has to offer a compensatory sacrifice if he omitted all three nights.

End quote from ash-Sharh al-Mumti‘ (7/358)

Each of you must feed one poor person for having omitted to stay overnight in Mina on that night.

And Allah knows best.