You should add active translators as members of the translation team, and remove them when they leave. Team members should have access to all of the project’s resources, and tracking their number is one of the ways for web-translators to determine the status of the team.
It is OK if a particular contributor wants to translate an article or two and does not want to be engaged with the team on a long-term basis. In such situations, there is no need to add her as a member.
It is a good idea to mark inactive members, for example if there is no interaction (bug reports, new translations, updates to existing translations, proof-reading) for at least six months. You can do that by unmarking the ‘On Duty’ checkbox for the respective project member under ‘Set Permissions’. Inactive members have absolutely the same rights as active ones—the only exception is that they don’t count for the total number of members, and they appear separately on ‘View Members’.