IMC, it is normal for families, especially families who can afford training for their children to group them together as friends who seek fortune together. This is almost madatory in nobility. A future noble's heir will usually be given childhood friends of other nobles and said group expected to support eachother later in life as staunch allies. Thus the children of the king, head priest, captian of the guard, and court wizard are brought up together and as they assume their hereditary roles, they are surrounded by known allies whose fates are entertwined. Noble families who have a child but no inheritance to pass along will often pick poorer children to raise with their child and give them training so when they all reach adulthood, their child has the support of others and is not alone as he has to find his way in the world.
So, it is usually assumed that the typical 1st level party at the beginning of the campaign all know each other from childhood and are capable of trusting each other. They might not have seen each other for some time but are typically considered friends. Such bonds are made usually around the age of 7 and if wealthy, raised under the same household till 14. At 14 they receive their individual training and once compelted (in a number of years) return to join together and get on with their lives.
Even if they aren't friends from childhood, it is usually recognised that there is a large "adventuring class", trained individuals who have no job nor inheritance. These people recognise the need of having a group with a range of skills for increasing their chances at succeeding at whatever they are trying to do or making them more employable to those with jobs for such a mercenaries. They they often gather togther, meet in bars, talk, boast, inquire, post flyers, etc. to finally gather together such a group. In large cities there are taverns that cater to such people by givign them a place to stay, eat, and make contacts. The owners of such taverns see them come and go and collect tales and gossip. A good word from such a tavern owner can even land a virtuous singleton in with a well established and possibly noble group (as there are many doors that are opened by even having one noble in the party). The nobility and rich also are known to go to such tavern owners to put togther a party of their patrons to prefrom various jobs.