It was late,and by that time they were feeling quite happy.
and 连接的两个并列句,用逗号隔开；
by that time 在后半句中式时间状语,后面是过去进行时.
One of them got in with his head at the top of the bed.
One of them got in 这是句子的主干,主谓结构；
with his head at the top of the bed 这一小节是with的伴随结构,做伴随状语.