It's possible to add or Invite more participants during a group video call provided their device supports video.
After Starting a meeting, in the window at the top left corner is the following icon. Click on it to access a drop down Invite window.
The following window will open. Search by name, extension or email.
Join Info option is the Meeting ID which can sent to a User by chat.
Call option is used to call someone during a web video meeting.
If a chat is opened with another user while in the conference there will be a button Add to call on the top bar which lets you add the user to the ongoing conference.