Just to clarify a few things for you: If you have owners who have 25% or more of the total unit entitlements, you can make a ‘qualified request’ to the secretary of the owners corporation to convene a general meeting. They must do this as soon as practicable, and in not more than 14 days. This is all in s19 of the Act.
You do not need the strata manager to convene the meeting. Although they likely have ‘delegated authority’ to act as secretary, the actual secretary can still convene the meeting. There has to be 14 days notice to owners.
For a meeting to be valid, it needs a quorum of at least 25% of the owners or of the total entitlements.
The ‘perfect’ solution might be a fully-fledged common property rights by-law. But you don’t need that. Per s112 of the Act, an owners corporation may grant a licence to use common property. It needs a special resolution, which means that not more than 25% of the votes cast are against the resolution, per s5 of the Act.
Note these percentages: you need 25% (in your case, 3 owners assuming all are roughly equal in unit entitlements) to require the meeting. You need at least 3 to attend the meeting. However many attend, if no more than 1 in 4 oppose the motion, it will pass. You don’t need 75% agreement.
So draft a motion along these lines: ‘That the owners corporation specially resolve to grant a licence to all owners to use the common property marked on the attached plan for X and Y, further, that the owners of lots A, B and C may use the marked portions as a garden or barbecue area…’. You get the idea.
Check all the meeting rules in Schedule 1 of the Act. It’s fussy, but quite intelligible.
Good luck!