A lot of couples are asking this:
Do you have to invite friends who invited you to their wedding?
Actually, the rule on reciprocating invitation and entertaining is strong. If the wedding took place just recently and you and your friends are still close, they should be on your guest list. However, in cases where your friendship has somewhat faded since their big day or if you are having a smaller wedding, it is okay to not include them in your guest list.
If you are inviting mutual friends, make sure they are aware of the restriction. This way, they won’t gush too much about your upcoming nuptials in front of the ones who were not invited and unintentionally create an awkward situation for everyone.