A: Your questions states that these members have joined a committee. If this is truly the case -- i.e., they were not appointed or elected to the committee -- then they are not officially members and should not be counted in calculating the quorum.

If they were appointed or elected, then the group that appointed/elected them may replace them by a motion To Amend Something Previously Adopted, which requires a two-thirds vote. If the President appointed them, he may remove them by his own action. (RONR pp. 487-488.)