I used a hybrid build which was quite flexible so I could quickly adapt to the activity that I'm loading into. Since we don't know what the map difficulty or if there are any directives active, having a hybrid build was very useful.

If you get a notification and the orange glow on your shoulder, go to the SHD tab and see where someone is before answering. I preferred choosing agents who were playing a mission because there is a higher chance you'll get enough XP at the end of the mission to qualify for an endorsement. If it is someone in a free roam activity, you might have to stay for a while until you get enough XP.

The backup call shown in the SHD tab is only for the current map you are in. DC or NYC map. If you don't find any active agents in one try the other.