It varies for my games, whether playing or running. If characters have overlapping backstory, I'll allow the fixer who calls the first ask them whether or not they know anyone who can do what the second guy does. Otherwise, always remember that the fixer is a fixer because he has a network of contacts. Every character should have some sort of contact; that's the route that the character gets sent to the meet, the fixer calling someone who suggests they call the character's contact, who calls the character and says, 'if you wanna make some money, be at Bluto's Bar on Berkley Bend at 9 PM tonight. Ask for Aaron.' It all comes from the same fixer, but it appears to be indirect, which is the point.
Another good way to do it is what was used in one of the Season 1 missions - the one where they're in the park, being guards for a fixer's meeting. Each fixer proposes someone (i.e. the PCs) in order to basically make sure that a) the meet is secure and b) they aren't screwed over by the other guys. Afterwards, well, PCs being what they are, they exchange contact information and 'hey, I got a Johnson on the line, and I need some of your kind of back-up. You in?'
Alternately - and this is one of my favorite ways to do it - everyone makes four or five different characters, essentially one of each sort of 'type' - combat, magic, matrix, social, vehicle. Those characters essentially make up their own small 'network' of associated runners, and depending on what the run needs and the player(s) want to play, they pull out a different character. The unplayed characters get some amount of karma/cash - 80%, for example - in order to keep up with the others, but there's a lot of flexibility with this sort of 'stable'.