Attenders are Friends who attend to the Spirit (however named) using the Friends Meeting for Worship as a vehicle for group (corporate) experience and worship.
If an attender is led by the spirit to seek membership status, she or he should write a letter to the Meeting's Oversight Committee. This triggers a process involving a Clearness Committee wherein the attender tests this leading, a kind of peer review process. See the Meeting's documentation for details.
However, it is not a requirement of Friends that attenders seek membership in order to fully participate in the life of the meeting. Committee work is open to all, and the label of "Friend" is used liberally to embrace all participants in the Religious Society who wish to claim some level of affiliation.
One's level of participation, more than one's status on paper, is often a better gauge of one's degree of commitment to Friends and their testimonies. Attenders may identify themselves as attending Friends e.g. "I attend such-and-such a meeting" whether or not they claim membership status.
Membership brings with it some new practices however. Members should expect their Meeting to be involved in carrying out their wishes upon death. What do you want to happen with your Flickr account, your Facebook, your email? What should happen with your body and so forth.
Attenders usually have not taken the step of asking a Meeting for these kinds of services, though I know of exceptions.
Membership, once acquired, may be transferred from Meeting to Meeting by a recognized process (see Faith and Practice or similar documentation). Other forms of membership, not obtained through any specific monthly meeting, may be available, either through a Yearly Meeting or perhaps FWCC. Check manuals for the latest practices or write to the clerk of one of these bodies for more information.
The choice to not pursue membership does not mean one is a second class participant in the life of a Meeting, only that one may have made other arrangements with regard to the dying process.
The practice around weddings is similar: one may have a Quaker wedding without being a member, or be a member yet choose to wed in a different manner, perhaps because other family would prefer to celebrate in a different tradition.
Sometimes a new Meeting will form from a worship group in such a manner that the attenders attain membership in the process of the Meeting itself becoming recognized as part of a larger group of meetings.
However, in the case of breakaway Meetings, affiliation with other Quaker organizations may come later if at all. Not all Friends recognize one another as Friends of their own kind (the division between unprogrammed and pastoral is an old schism, however there are others as well).
Attending Friends may choose to not seek membership because their affiliations extend to other traditions, some of which may be controversial among Friends.
A given membership may collectively fear infiltration by "pagans" or "spies" or "gangs" or "ranters" or "the AFSC" or "clerics" some other persuasion judged "not Friendly enough".
Rather than rock the boat by insisting on membership, attender status may be the way to go i.e. continuing to participate as an attender may be the most "sympatico" or mutually agreeable outcome for a given special case Friend. Ben Franklin might be considered a role model in this regard.
Note that a common practice among Friends, historically speaking, among members and attenders alike, is journaling, which these days might mean blogging.
If you're looking to adopt more Friends' practices, consider taking up journaling in some way shape or form. More and more people are doing that anyway, which might be taken as a sign that Friends were on to something early -- which fact should probably not seem surprising given Friends attend to the leading of the spirit or zeitgeist.