Bring a Friend (BAF) is Mythic's name for one of the ways that the game generates adds. When you pull a mob that BAFs, extra mobs come with it, provided that the puller is grouped with enough other players to trigger the BAF code.

(Actually it's a little more complicated than that. For BAF purposes, the game counts not only players in the puller's group, but also any other players and groups that assisted the puller's group in fighting mobs during the last few minutes.)

Only some types of mobs are capable of BAF'ing.

The number of adds that get generated by the BAF code is partly random, but the following table gives a pretty good idea:

This many players...   usually get this many mobs when they pull one.

1 1

2 1 or 2

3 or 4 2

5 or more 3 or more

BAF'ing is not the only mechanism by which adds are generated in the game. Adds can also come due to linking, calls for help, and scouts.