Formal:
* Well-informed
* Knowledgeable
* Up-to-date
* In the loop
* In the picture
Informal:
* In the loop
* In the mix
* In the know-how
* In the swim of things
* With your finger on the pulse
* In the thick of it
More colorful:
* On the inside track
* privy to the secrets
* in the know-how
The best substitute will depend on the context and tone of your writing.