Riding in single line Is it safest for a group of cyclists to ride in a single line?  Is it required for cyclists to ride in a single line in a narrow bike lane, or can they pass on the left in open traffic?
In general it depends on the road or trail, how wide it is, how straight or twisty, how visible the trail ahead is etc.

On a wide trail without tight turns and plenty of forward visibility riding two-abreast is fine. 

On a narrower trail with tighter turns or less visibility, riders should ride single-file and stay on their side of the trail. Common sense applies to overtaking, i.e., don't try to pass if you can't see far enough ahead.

There may be laws that apply to cyclists in different locations of course.