If you select Joint Approval option ,everyone who has approve access to the document must approve the document to the next stage before the document is moved. Generally, only one approver needs to approve the document to the next stage. If, however, you have activated Joint Approval, then all users with approve access must approve it. The exception to this is, if a group is listed as having approve access, only one group member needs to approve the document before it is moved.
If you have a single person and a group both defined as having approve access to the document and if you have Joint Approval activated, the single person AND one member of the group must approve the document before it can move to the next stage
Group1 members :::::: grp1memeber1, grp1member2, grp1member3
Group 2 members :::::: grp2member1, grp2member2, grp2member3
Individula Approvers :::::: individualApprover1, individualApprover2
If you add the Group1, Group2 , individualApprover1 and individualApprover2 as Approvers and if you select the "Joint Approval" option. Then
anyone from Group1 (any one of grp1member1,grp1member2,grp1member3) and
anyone from Group2 (any one of grp2member1,grp2member2,grp2member3) , individualApprover1 and individualApprover2 has to approve to move document to next stage