There are two common patterns when assigning group membership in bulk: putting one person into several groups, and putting several people into one group. Both start in the same place: Management → Contacts.
Roles are per-group. The role you pick in these flows applies to every group you select in this action. To assign different roles in different groups, repeat the flow with a different role each time.
Add one user to several groups
Open the contact. Go to Management → Contacts and click the user's name to open their contact card.
Start the group flow. Click Add to group.
Pick a role. Choose the role this user will have in the selected groups (Player, Staff, Admin, or any custom role you've defined).
Select the groups. Tick every group you want to add the user to.
Confirm. Click Done.
The user is now a member of every selected group with the role you chose. If they were already in some of those groups, those memberships are left as-is and the new memberships are added on top.
Add several users to one group
Open Contacts. Go to Management → Contacts.
Select users. Tick the checkbox next to each user you want to move.
Open the Actions menu. Click Actions, then Add to group.
Pick a role. Choose the role these users will share in the target group.
Pick the group. Select the destination group.
Confirm. Click Done.
Every selected user is now a member of the chosen group with the chosen role. Pre-existing memberships in other groups are not affected.