Yes, the team may build the robot with any attachments they want to have. The only restrictions are on what types of elements can be used (Rule 4), and that the robot and anything it is touching must fit completely within Base before the robot is started or restarted (Rule 29).
There is no Robot Game rule that requires more than two team members to handle the robot. Rule 2 allows tagging in at any time, "as desired" by the team.
In order to be eligible for an award, each team member must participate in the Project judging session in some way. All team members present on the day of the tournament are expected to attend the robot matches and the judging sessions, of course, but FLL does not specify what roles each team member should have.
Several of the skill areas evaluated on the Core Values rubric are relevant to this discussion. Different teams will have different skill levels in the various areas, which is quite natural. (Different FLL volunteers have different preferences, too!)