Select what base table to use (user, content etc). Add relationships to get the relevant data, like "User: Group membership" to get the groups a user is a member of and then use "Group membership -> Group membership: Group gid" to find the group fields, name etc.

Example: If you want a list of nodes with a field saying what group they're in, start with a node view and add a "Node: Parent group" relationship to the group they belong to.

Example: If you want a list of groups with one field listing the amount of nodes in the group, start with a group view and add a relationship to "Group: node content" and work with that.