The "just work on a open-source project" advice isn't that bad, but you should pick something that you would benefit from improving. It is still a good idea to solict others to help you in a physical meeting, however. Find people who would benefit from improvement in the same project.
I think rather than a website or webapp, what you are looking for is a computer club. I go to a weekly meeting in Austin of ALE, a linux users group. We meet 7 pm to 11:30 pm, and we are an "experimental" group, in that we never have presentations or a planned agenda -- people bring computers with problems and we fix them, new people show up asking for help learning linux, etc. There are about 5 people there who come regularly with programming issues, ranging from side-business startup sites that are in php and mysql to hobbiest robots.
If you are really serious about this, find a place (possibly your house or apartment) and have a "Saturday afternoon hack-a-thon" every week or every other week. If you provide food and coffee people will come. Your main problem is likely to be keeping non-programmers from showing up and just talking.
If you have trouble finding a place to do this, see if you can locate a co-working or "jelly" type place in your area. A co-working place might let you use the area during non-business hour such as on weekends, and the jelly people probably know all the good coffeehouses.