In a statement of claim, the shareholders allege Vocus also breached its continuous obligation requirements by not confessing to its missed profit forecast to investors earlier.
Any person who purchased Vocus shares between November 29, 2016 and May 2, 2017 will be eligible to join the claim.
Vocus shocked the market in May 2017 when it announced it expected its full-year profit for the year to June 30, 2017 to come in at between $160 million and $165 million — a significant drop on its earlier forecast profit of between $205 million and $215 million for the same period.
Vocus shares fell about 27 per cent in the days following the downgrade.
At the time, Vocus said its downgrade was caused by the revenue it expected from a cohort of large projects being received slower than it had expected.
The company also cited the impact of lower than forecast billings from its customers and an increase in delivery costs including the need for additional staff.
The downgrade came amid executive upheaval at Vocus following a failed tilt by founder and then director James Spenceley and Amcom chairman Tony Grist to spill the leadership of the group.
Mr Spenceley and Mr Grist resigned from the board after Vocus resisted their spill push.