I don't buy Xenon's statement about it needing to be expressly allowed
Ouch.... are you aware of what you are really suggesting...?
It is clear that you are allowed to use assist in ritual spellcasting (p. 296), group summoning (p. 300), assist in the healing process (p. 208), to help find a buyer when fencing gear (p. 418), as perception when several people looking for the same thing (p. 135), when tracking someone (p. 49), for animal handling to assist a critter's tracking test (p. 137) etc. Common for all of them are that they are all actions you normally perform out of combat and out of combat turns.
In addition to "regular" teamwork tests you also have specific uses of teamwork that have rules of their own. Leadership can be used to direct a subordinate to help them on a composure test or their next action or to inspire a subordinate to help them on a surprise test (p. 142). Diagnostics when someone use or repair an electronic equipment (p. 257). There is also a variant of teamwork test that may be used in melee combat (p. 188, but so far I have seen nothing similar for ranged combat).
With the exception of Leadership and the explicit Melee Teamwork test I think you need to be
very careful before straight off applying standard teamwork tests (increasing limits and adding dice as a positive dice pool modifier) to actions that are normally resolved in action phases during combat (such as regular spell casting, ranged combat, throwing grenades, sprint tests, etc etc).
There are already several mechanics in place for helping your team in combat as is;
SR5 p. 189 Defender has defended against previous attacks
If a character has defended against at least one other attack since his last Action Phase, apply a –1 cumulative modifier for each additional defense roll.SR5 p. 179 Suppressive Fire
Anyone in the suppressive fire zone or immediately adjacent to it takes a dice pool penalty to all actions equal to the shooter’s hits, unless they are completely unaware of it (a magician using astral projection, for example).SR5 p. 177 Attacker in melee combat
This modifier applies to any characters trying to make a ranged attack while in melee combat.SR5 p. 187 Friends in Melee
Whenever an attacker has at least one other person in the melee with them, they gain a +1 dice pool bonus. The bonus does not increase based on the number of friends.Also, if you want to increase the limit on spellcasting you can use reagents and if you want to increase the limit on ranged combat you can use a smartgun..... Or edge.
I would would think twice before allowing breaking the limit with assistants if I were you.