Answer: It is not permissible to combine two prayers unless there is a lawful, Shar‘i excuse—such as travel, an illness that makes combining necessary for the sick person, or rain. These are the circumstances that allow combining. To combine without any such excuse, as you have described, is impermissible.

As for your question about whether you should pray with them when they do this or not:

As we have already explained, this form of combining is invalid. Therefore, do not join them in it. Pray the first prayer with the congregation, and when they rise to perform the second prayer, do not pray with them. You must also speak out against their action and clarify to them that it is forbidden. If they refuse to heed the advice, then pray the first prayer with them and do not pray the second.

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Shortening and combining during travel

Question: I am part of a group that has been assigned an official, temporary duty. The nature of the work has often required us to combine and shorten our prayers. We have now been on this assignment for about one and a half months and are still in this situation. We do not know how long we will remain. During this entire period we have been unable to attend Jumu‘ah, even though we have enough people to establish it. Please tell us: during this time are we considered travellers, and does the traveller’s ruling apply to us or not? If we stay for a long time, may we continue in this manner? Kindly advise us, and may Allah reward you well.

Answer: If carrying out the duty you mentioned is outside your home town,