Additional tenant to AST within 6 months

Tenancy Types and Management

Have a tenant with AST signed less than 2 mths ago in a furnished flat.
Now asked for a work colleague to live with her as problem finding local accommodation.
Both are on work permits with differing expiry dates.
Company acting as guarantor for existing tenant, not second (different country and employment type). Company have paid 6 mths rent and deposit in advance
Using RLA AST and Deposit Guard.
Does adding new tenant (jointly and severally responsible) materially change the AST to the point of needing new AST, deposit scheme and inventory?
If yes, can I make charge to cover costs redrafting etc.
If new AST, I will ensure company underwrites.
Is there anything else I should do to protect tenancy.