。
I was so happy to be able to meet you at the offline meetup.
Literal
Offline-meeting [at-では] honorific-meet-can was-happy [copula-です].
Internet-community register. オフ会 is net-slang for 'offline meeting' — an in-person gathering of people who normally interact online. お会いできて uses the humble pattern お + verb-stem + できる 'be able to do V (humble),' showing deference toward the person one met. The ~てXかった pattern (here お会いできて嬉しかった) links the cause (meeting) to the emotional result (happiness). 嬉しかったです is the polite past form of い-adjective 嬉しい.