I would suspect that there are a fair amount of colonies that are members, but that there would be limits on membership based on population (has to have a certain amount of sentients) and the government type (if a colony has a government form or commits certain social habits that are faux pas with the UFP, they don't get in...)
Using these parameters, most colonies wouldn't qualify and would fall under the control of the Bureau of Colonial Affairs or would be independent worlds. Older, better settled ones like Mars, Ejul Thelni, Cimera III, etc. would be full member worlds.
"War is an ugly thing but not the ugliest of things; the decayed and degraded state of moral and patriotic feelings which thinks that nothing is worth war is much worse. A man who has nothing for which he is willing to fight, nothing which is more important than his own personal safety, is a miserable creature and has no chance of being free unless made and kept so by the exertions of better men than himself."
John Stuart Mill