The Demolition Team is a group of mercenaries whose primary group of expertise is destruction of buildings. 
At some point in time the team was formed with Rosie as a leader. It worked all over the world with such organizations as GRU, IRA and Mossad before eventually starting to work for Damien Darhk and H.I.V.E..
They tried to kill Quentin Lance by destroying the building in which he was drawn, but that attracted the attention of Team Arrow, which tracked the group down to their hideout. Demolition Team managed to escape, destroying the hideout and lacing their laptop with nails. 
Later they've tried to destroy the building in which mayoral election debates between Oliver Queen and Ruvé Adams were held, receiving a direct order from Ruvé to kill Oliver, which they were not able to fulfill being neutralized by Team Arrow.  Afterwards they were arrested by the SCPD.
Behind the scenes
- In the DC comics the Demolition Team is also a group of construction-themed mercenaries led by Rosie.