What do you say to those who believe they can lead a spiritual life without attending a weekly service or some kind of group fellowship?

​I would ask them if they also believe they can live a healthy physical life without attending regular meals.
It is in the context of the weekly worship service that Christians are strengthened for our daily walk. The Lord knows our weaknesses and our needs, and so established this pattern. It is there that his people do many of the things that publically mark them out as God’s people, and by which they are reminded and learn many of the things necessary for the Christian life.
As we publically and corporately give thanks to him, call on him in prayer, sing praises to him, and have him speak to us through his word we acknowledge his glory and our dependence upon him.
Christians who think they can, or try to, neglect that great resource will find themselves missing out on those blessings and outlets for spiritual service, and suffering from spiritual malnutrition.