Standing alone in the saff
In the name of Allah, the most Beneficent, the most Merciful.
It is Makruh to stand alone in the saff where there is space in the front saff. However, if the front saff is full then he should wait for someone to come, until the ruku position. If no one comes, he should then move someone back from the right or left side of the front saff. It is preferable to move back someone who he thinks knows the aforementioned ruling. If he thinks no one knows this particular ruling and by moving someone back it may lead to the person breaking his salah, he should stand alone in the saff and commence his salah. (Raddul Muhtar p.310 v.2)
With regards to your question if someone pats you on your back, gesturing you to move back then without excessive movements (Amal Kathir) move a few steps backwards and join with the person. The people in the front saff will come together without amal kathir.
Only Allah Knows Best
Mohammed Tosir Miah
Darul Ifta Birmingham.