A group is a label you give a set of customers so you can find them, and message them, together. BeNotely already knows who's due back and who hasn't been in for a while; groups are for the things only you know.
Making one
On a customer's profile, under Groups, press the plus and type a name such as Colour club. The group exists from then on and shows up for everyone else.
Adding people
From a profile, add them to any group. From the customer list, tick several people and add them to a group in one go.
Using a group
Filter the customer list by a group to see just those people. Start a campaign for the group and only they receive it; see Campaigns. Remove someone from a group from their profile; delete a group and nobody is deleted with it.
What's the difference between a group and a segment?
A segment is worked out for you from what people do, such as "haven't been in for 90 days"; it changes on its own. A group is a list you keep by hand. Both can be messaged.