Overwatch – Building Mei’s Ice Wall on Top of Another

Not sure about console but on PC at the top of your friends list there is a 'Recent players' tab.

If you right click on a person you will see an 'Add friend' option in the dialog menu which, provided that they accept the request, adds them to your battle.net friends list.

You can also right click on a person's name in chat and add them that way (as pointed out by GnomeSlice below).

As a rule of thumb, no.

The situation may differ depending on whether you're attacking or defending, but the time it takes to die, respawn and run back is time you could be spending diverting enemy attention, rounding up health packs (to zone/deny your enemy getting them when the next team fight starts), or scouting for teleporters/turrets/flankers.

One thing I will mention though, is that in a situation where it's suicide or die to the enemy without the option of retreating/regrouping (let's say you overextended and can't backtrack without getting pipped by a turret or sniper, and you're watching a Reinhardt trudging towards you); suiciding is the better of the two options.

This is because if you let that Reinhardt hammer your face in, he gets ultimate charge, which stacks the odds against your next engagement. If you suicide, the respawn timer is the same, and he is ~10% further from his ultimate because you decided the cliff would treat you better than his beatstick.