which sentence is correct?

I'm happy to see my friends again.

1)また友達に会う嬉しいです。

2)また友達にうれしいあいます。

Thank you.

Neither. You cannot string up the words just like that.

You can say:
また友達に会うのがうれしいです。(I am happy to see my friends again.)
また友達に会えるのがうれしいです。(I am happy to be able to see my friends again.)
また友達に会えてうれしいです。(I am happy to have met my friends again. - After or upon meeting the friends again)

The construction is that you need to turn the verb 会う into a noun phrase by using "koto" or "no". I wonder if you've learned that bit of grammar.
[また友達に会うの]が　うれしいです。


Similar structure is used when you want to say things like "I like [doing something]."

Start with the sentence to say "I like [something].":
私は[日本]が好きです。I like Japan.
私は[すし]が好きです。I like sushi.

What goes into [ ] needs to be a noun/noun phrase.

So you need to put [traveling in Japan] in a noun/noun phrase. Then you use "no" or "koto."

日本を旅行する・します I travel in/through Japan.

[日本を旅行すること]が好きです。
[日本を旅行するの]が好きです。

well explained :D

Thank you very much.
I have learnt some, but I can't remember them. But I'm trying to review again.