Route a contact through a flow’s logic using a ‘Split by Group Membership’ node. This will allow you to send customized messages to a contact depending on their groups.
For example, you could stop contacts who’ve already completed a previous registration flow where they’ve already been added to a group from restarting all over again:
By splitting on the group membership only contacts who aren’t already added to the ‘Completed Registration’ group will continue in the flow. Those who are added to the group will receive a different message letting them know they’ve completed the separate registration flow.
You can add or remove contacts in a group either manually or via a flow.