To create a group, click the + button in the main screen. You will be presented with a panel that allows you to choose among your contacts who to add to the group. If you want to invite people to the group that are not your contacts, invite them to be your contacts first.
The Group members field has a search. Start typing, and you will immediately be presented with matches among your contacts.
Anyone can create groups. Free accounts have a limitation on the number of groups.