If the church or registry office (or other venue) is small, or if numbers are simply too tight, then it is acceptable to invite some guests to the reception alone, or just to evening celebrations.
A note should be placed inside the envelope with the invitation, for example:
Owing to the small size of St John's Church it is possible to ask only very few guests to the service. We hope you will forgive this invitation being to the reception only.
If guests are just invited to celebrations later in the day, then it is best to get some separate evening wedding invitations printed:
Mr and Mrs John Debrett
request the pleasure of your company at the reception
following the marriage of their daughter