From the Contact's Information Page

To add a contact to groups or remove them from groups, go to their contact information page and look under Details on the right-hand side:

Choose groups for the contact from the Groups drop-down, or remove a group by clicking the x in front of its name. Each change is saved as soon as you make it, and the groups the contact belongs to appear in the Groups field.

Using a Flow

You can add an Add Contact to a Group or Remove Contact from a Group action to a node to move contacts between groups as they go through the flow. In the action, choose the groups under Groups. Here, the action adds the contact to the "New Students" group:

In the example below, a Wait for Response node sends students who will live on campus to an Add to Group action that puts them in the "On Campus" group. Students who will live off campus go to a second Add to Group action that puts them in the "Off Campus" group.

We do this by splitting contacts on their reply, which is saved as "Housing". The has any of the words rule puts any reply that contains "on", "dorm" or "residence" in the "On Campus" category and any reply that contains "off", "apartment" or "home" in the "Off Campus" category. Each category leads to its own Add to Group action.