Any answer to this question would necessarily be a ruling, interpretation, or series thereof. Unlike a few other places like Melee Teamwork (CRB pg. 188), it's not specifically called out as part of Matrix actions, so I can only tell you what's worked for me, with the reasoning below:
A) Teamwork should apply everywhere unless otherwise specified. Standard, common-sense requirements that you be able to help still apply.
Teamwork Tests (CRB pg. 49) posits Teamwork as a core concept of Shadowrun, at the very least in-universe. The example given of tracking someone as a Teamwork action would normally be either a Success test, or an Opposed test (see Using Tracking, CRB pg 137), but apparently is fine for a Teamwork test in either of those cases. Using Tracking doesn't specifically say whether Teamwork rules apply, other than specifying both that they do and how they should when the tracker has a tracking dog helping out.
Melee Teamwork (CRB pg. 188), Ritual Spellcasting (CRB pg. 295-296), and Perception (CRB pg. 135) also specify what appear to be added restrictions to the Teamwork mechanics in those contexts, rather than explicit and necessary permission that "yes, you can use the Teamwork rules here."
B.I) Only the 'leader' receives direct benefits or damage of a roll
The text of Teamwork Tests simply does not address this issue. However, mechanically speaking, it is only the leader's roll that decides the final outcome. This is reinforced in Melee Teamwork, where 'hits' do not do damage, but rather add to the dice pool of the "eventual attacker". Failures on helping rolls, though Opposed, are not called out as having any consequences.
B.II) House rules on mechanical repercussions - parity with meat-space, and a little digital spice
As an optional house rule, the Melee Teamwork rules can be adapted into Matrix decking - specifically, the part about rolling an Opposed check vs a given stat/skill of the defender. This sets a not-entirely-arbitrary success threshold for helpers.
In addition, for the purposes of the Illegal Actions section (CRB pg. 231) as well as determining whether helping is a Simple or Complex action, I would deem any character participating in a Teamwork action to be actually doing the action themselves. In effect, since "all Attack and Sleaze actions are illegal", any character rolling against a set difficulty or active opposition to help such an action would have the chance of repercussions for failure of their individual help roll.
This would be narratively justified as the other characters fuzzing or whatever else you do to overwhelm an object's defenses, so that someone else could slip through. In meat-space, anyone attacking or threatening an attack is obviously hostile to the defender, but the hard part is responding to or communicating so many threats. In the Matrix, the hard part is distinguishing attacks from legitimate or simply error-ridden traffic. If traffic from any given source is deemed to be hostile, notifying the local DemiGOD is a built-in feature for Matrix security. This applies to Sleaze rolls at the very least, though I have no further justification for Attack rolls other than that it seems ok with my players.
This can tend toward only Matrix-capable characters attempting to help in these rolls, but this, I believe, further suits the nature of the Matrix and the "avoid attention at all times" hacker/decker ethos.
B.III) My (limited) understanding of the Matrix leads me to believe other characters could help to lower an object's defenses, but it would still be the 'leader' placing the mark.
Recognition Keys (CRB pg. 236) gives the reasoning behind other people not being able to use your mark as the following:
Your marks are specific and connected to your persona and whatever you’ve marked, so you can’t just give them out for others to place or transfer them to other people.
Since this is not constrained by a Teamwork section as reasoned above, I would allow this under my current understanding of the Matrix's world.