To Add a Contact or multiple Contacts to a Group

Select the Group via Contacts > Groups

Select the Group you would like to add Contacts to.

Click the Add Contacts button

In the search field search for the group/s or individual contacts that you would like to add and select them. Select your Contact(s) and Add (green button)

To remove a Contact from a group click on contact's name and then click on the Actions drop down and select Remove from Group option.