The simplest answer would be - wherever they are needed. But at least once a week by a team email.
Furthermore, the maintenance of constant contact:
- Regular e-mail
- Internal SFI chat
- SFI internal e-mail
- An invitation to participate on your blog / website
- Invitation to participate in co-op
- Invitation to participate with you on social networks
- To organize incentives and competitions
In brief, to hold their attention constantly occupied, but be careful not to overdo that, the secret is to encourage the initiative for them to act according to their desires and intentions, rather than imposed.