Please see the forms and information at http://www.tug.org/join.html. You can join online, or by filling out a paper form. The NTG (Dutch) and UKTUG (United Kingdom) TEX user groups have joint membership agreements with TUG whereby you can receive a discount for joining both TUG and the local user group. To do this, please join via http://www.ntg.nl/newmember.html (the NTG membership page) or http://uk.tug.org/Membership/ (the UKTUG page), respectively, and select the option for joint membership.
Each year’s membership entitles you to the software and TUGboat produced for that year (even if it is produced in a subsequent calendar year, as is occasionally the case). You can order older issues of TUGboat and TEX memorabilia through the TUG store (http://www.tug.org/store).
The current (2015) TUG membership fee before March 31st is $85 (US) per year for individuals and $55 for students, new graduates, seniors, and citizens of countries with modest economies. Add $20 to the membership fee after March 31 to cover additional shipping and processing costs. The current rate for non-voting subscription memberships (for libraries, for example) is $105. The current institutional rate is $500, which includes up to seven individual memberships.