To add a contact to a group via the contacts tab, navigate to the "contacts" tab, check the box to the left of the contact whom you'd like to place in a group, then click the group icon that appears above your contacts to select a group or create a new one:
To remove a contact from a group via the contacts tab, click the group from you which you want to remove your contact, check the box to the left of the contact whom you'd like to remove, then click the "remove from group" icon that appears above your contacts:
Using a Flow
You can attach an Add Contact to a Group or Remove Contact from a Group action to a node to move multiple contacts between groups instantly.
In the example below, we're using a Split by Contact Field action to direct contacts who reside in Nigeria's northern states to an Add to Groups node that places them in the "Northern States" group.
As seen in the flow:
We achieve this my splitting our contacts on the "State" contact field. The has any of these words response rule allows us to create a rule that places any contacts whose "State" field contains any of the words "Adamawa", "Bauchi", "Benue", etc. in the "Northern States" category, which leads to the Add to Groups action above.