It’s pretty simple to tell which other assistants are organized and friendly — if you have an easy time scheduling meetings with someone, it’s probably because he’s a great assistant. Other assistants are evaluating you by the same criteria, so if they’re enthusiastic about meeting you for drinks, it’s because they respect you and want to build an even stronger working relationship. You should always take other assistants up on an offer to meet in person and make the effort to reach out to people who interest you — these peers are going to be your biggest champions over the long-term. When it comes time for them to refer you for a job, they’ll actually have some basis for a recommendation, since they've worked with you and also gauged your personality IRL.
Realistically, you’re not going to be able to meet with every person on your contact list, and extending your already-long work day and too-tiny salary to get drinks can seem daunting. Still, networking is how most people move up in this industry, and the people you befriend when you're both at the bottom will be your tried and true allies as you climb to the top. So the next time you schedule drinks for your boss at a bar you're dying to try, ask the assistant on the other side of the Outlook calendar to try it out with you sometime. It's worth it.