It is notoriously hard to keep track of the X-Men.
With over a hundred characters that could be considered members or affiliates, a half dozen teams at any time, and constantly rotating line-ups; this isn't exactly surprising.
This timeline attempts to help. I've tried to show membership on all the major teams by YEAR. New members are in bold, with the (month) the joined beside them (note that they may have been in the books before, but weren't considered officially part of the team). Characters that left a team during any particular year are marked with italics, also with the (month).
Since many characters leave for a few issues and then return in the same year, I've tried to only note absences that are mentioned in the books to be quitting the team, or taking a vacation. I've also marked short breaks as (month to month) - either the months they were missing, or the short periods they were on a team.
Also, I try to keep the team leaders at the top of each grouping, and associated characters that aren't actually teammates at the bottom, but I don't think I was very rigorous about it.
I'm sure there will be lots of ways that this is unclear, so let me know on my Tumblr or gmail if you have a suggestion. (I am always kiragecko, no matter where I go.)
1963-1979 (Note: pretty boring)
2000-2016 (Note: recent years may never be finished)